Do you have a video so we can see how bad it is? You should try to find a low cost vet in your area, at least try calling around and explain your situation. It could be something as simple as a strain or sprain, something a little worse like a tear or a broken/fractured leg. But no one here will be able to help without at least a video. A video would help those more experienced, but a vet would be the best option to tell what is it, and to give the proper pain meds, as your bun could be in serious pain even if it isn't broken/fractured, but some kind of muscle tear. Depending where you live a lot offer "care credit" which is 0 percent APR up to the first 2 years, so no interest. But you have to pay by whatever date you have set otherwise interest goes through the roof haha. I used this on my dogs emergency pyometra surgery since it was about 6k. Got it payed off in a little under a year, no interest. Source
"Our No interest if Paid in Full Within 6, 12, 18 and 24 month financing options (sometimes called Deferred Interest) let you pay for purchases of $200 or more with minimum monthly payments made throughout the duration of your promotional period.
For Deferred Interest/No Interest if Paid in Full promotional financing options, no interest will be charged on your promotional balance if you pay your balance in full by the promotional due date. If your promotional purchase is not paid in full by the end of the promotional period, interest will be charged to your account from the purchase date. "
care credit link
But a video would really help if you could post one. Best wishes to you and your bun though, hope it's nothing serious.