One of the ways that Darcy displays his love for Elizabeth is when he tracks down Lydia and Wickham following their elopement and pays the expenses for their wedding. We don't know how much exactly he had to spend, beyond Mr. Bennet saying Wickham would be a fool if he took less than ten thousand pounds in chapter 49. However, I think that the actual amount that Darcy paid out would have been less than to 3,500 pounds based on the calculations and assumptions below.
What is known:
Wickham's debts amounted to considerably more than 1,000 pounds
Darcy provided 1,000 pounds to be settled on Lydia.
Darcy purchased Wickham's commission, an ensigncy in the Regulars.
Commissions for the rank of ensign officially cost between 400 or 600 pounds depending on if it was a regular infantry regiment or the more prestigious Foot Guards.
Mr. Bennet was to provide the Wickhams with 100 pounds a year during his life and to provide Lydia with a 1/5 share of the 5,000 pounds settled on Mrs. Bennet following both the deaths of both Mr. and Mrs. Bennet.
What is or can be supposed:
Darcy had to bribe to Mrs. Younge in order to find out where Wickham Lydia were hiding.
Darcy likely had to pay a premium to obtain a commission so quickly.
Mr. Bennet paid Wickham's Meryton debts or made arrangements to do so before he found out Darcy was involved. 
Darcy paid for the marriage license at a cost of 10 shillings.
Darcy paid for the express that Mr. Gardiner sent Mr. Bennet.
Darcy paid the cost of moving Mr. and Mrs. Wickham from London to Newcastle, for which I will budget 100 pounds. 
Wickham would have attempted to get an additional amount for himself. 
How this all adds up:
1000 pounds to be settled on Lydia
1500 pounds to pay off Wickham’s debts (I’m estimating)
400 pounds to pay for Wickham’s commission
100 pounds to bribe Mrs. Younge (note: I think the amount was probably smaller)
So, in conclusion, based on the evidence in the text and my own historical research, I would say that Darcy did not pay out anywhere near 10,000 pounds in order to bring about the Bennet-Wickham wedding. Which isn't to say that the amount that the amount he did pay wasn't significant. It was considerably more than many a gentleman's yearly income and certainly more than the Bennets would have been able to afford.
 In Chapter 50 Mr. Bennet later writes to Mr. Gardiner, begging him to tell him how much was actually spent, indicating that Mr. Bennet didn’t know and was just throwing out a number.
 Chapter 52, Mrs. Gardiner’s letter to Elizabeth, it is unclear whether that number is all of Wickham’s debts or just his non-Meryton debts (note 9), either way I would say that the sum of his debts would be less than 2,000 pounds or Mrs. Gardiner would have used that number in her letter.
 Chapter 52, Mrs. Gardiner’s letter to Elizabeth.
Chapter 50, a letter from Mr. Gardiner to Mr. Bennet and Chapter 52, Mrs. Gardiner’s letter to Elizabeth states that the commission was purchased.
Brian Southam, Jane Austen in Context, ed. Janet Todd (Cambridge University Press, 2005) 373. According to this source, the standard fee for an ensign’s commission in an infantry regiment (which had the least expensive commissions) was 400 pounds, followed by the Foot Guards where an ensign’s commission cost 600 pounds. Calvary regiments cost more (between 735 and 1600 pounds) but do not have the rank of ensign. I will also note that in times of war it was not always necessary to purchase a commission if you were not picky about which regiment you entered.
 Chapter 49, a letter from Mr. Gardiner to Mr. Bennet.
 Chapter 52, Mrs. Gardiner’s letter to Elizabeth, complete supposition, but it seems likely that a person who would be friends with Wickham would be willing to betray him for money. I cannot say how much would be a reasonable bribe, but my guess is that it would be less than 100 pounds (which was a very large sum back then).
 My research hasn’t turned up exactly what amounts the premiums were, since it does indicate that during the Peninsular War (1808-1814) most young officers were able to get their commissions for free, I will make the assumption that the premium was no more than the difference in cost between the official cost of Wickham’s commission and the next most expensive commission; this was 200 pounds in the case of infantry to Foot Guards, 135 pounds in the case of Foot Guards to Dragoons (whose lowest rank was a cornet).
 In chapter 50, Mr. Bennet writes to Mr. Gardiner and begs him to tell him how much money Mr. Gardiner spent in bringing the marriage about so that Mr. Bennet could know the extent to which he was indebted to his brother-in-law to whom Mr. Bennet feels an obligation to repay. Mr. Gardiner doesn’t send this information but later in the chapter asks Mr. Bennet to make assurances to Wickham’s Meryton creditors that they would be repaid soon. Due to Mr. Bennet’s feelings on the matter of being in debt to Mr. Gardiner, I think it is reasonable to assume that Mr. Bennet assumed those debts himself.
 Joan Elizabeth Klingel Ray, Jane Austen for Dummies (Hoboken: Wiley Publishing, Inc., 2006), 144. Jane Austen for Dummies states that an engaged couple “could purchase a [common/ordinary] license from a clergyman for about 10 shillings.” While this is not explicitly mentioned as a cost in chapter 52, Elizabeth receives a letter from Mrs. Gardiner which states that “[n]othing was to be done that [Darcy] did not do himself[.]” I am assuming that the marriage license fee counts as something.
 See note 10, though this is something which I think it is slightly more likely that Mr. Gardiner paid for himself without telling Darcy at the time as Darcy was not necessarily present when Mr. Gardiner wrote the letter.
See note 10, this is another cost which I think it is likely that Darcy covered mostly because I think Wickham at some point whined “But how will I get to Newcastle?” LeFay states that traveling post would cost at least one shilling per mile while Family Fortunes puts the amount at one shilling nine pence, Newcastle approximately 283 miles from London and extrapolating that the Wickhams would end up with their costs being closer to 1 shilling nine pence I came up with 24 pounds, 15 shillings and 3 pence. As tolls and the cost of overnight lodging would be extra, I rounded up to 25 pounds and multiplied by 4 to get 100 pounds, though I would guess that for a reasonable person in the Wickham’s situation, less would be entirely adequate.
 This one is pure conjecture on my part, but I do think that it is likely that Wickham attempted to weasel a small amount for himself, though my guess is anything like that he received would fall under an aggravated “here is some money for your travel expenses.” If there was anything else that Darcy did, I think it would be more likely to be a (written) promise that Darcy would later purchase Wickham a promotion or would provide the Wickhams with an additional 50 pounds per year following Mr. and Mrs. Bennet’s deaths.
 Again, I picked a number that seemed excessive to me, particularly when you consider that most people in Regency England lived on less than 100 pounds per year. This amount would include Darcy’s travel costs, and the legal costs associated with the marriage settlement, etc.