Yeah, I think $2500 is unreasonable. If you like and trust this broker, ask him why the deposit is so high. If he can't clearly explain, find another source for your financing.
I am a big believer in the concept that you don't get something for nothing. I have never had an issue with a serious client paying me a $400 deposit to lock a rate. In the event the appraisal comes in low, or the loan is denied, I will always refund this deposit. But I feel that if I'm going to secure a rate for a client, and the lock fallout would hurt our company if the borrower deliberately walked from the deal, a deposit is fair.
One reason so many brokers were forced out of the business in 2007 and 2008 is that their lock delivery ratio was low. Too many of them did not respect the value of a lock commitment and this hurt everyone else. Those left in the business have a strong motivation to deliver their locks and deposits help insure that they will.
You have too many choices out there. Paying $2500 as a deposit is, in my opinion, a good reason to keep looking. You can do better. Negotiate or switch.
Best of luck and let me know if I can help.