I believe I stated what it cost to have the tumor removed from Pegasus and to have it biopsied: $273 US Dollars, total. (Vet in Raleigh NC)
From your description I strongly suspect that Pegaus's tumor and Bun's tumor are identical. Personally I'd find another vet who isn't looking to you for the next payment on his Lamborghini.
If I had it to do over, I would use the very same vet. I would have insisted that they amputate if they weren't 100% certain that they could get all the cancer. If they didn't amputate I would have scheduled visits every month or two to examine him to make sure it hadn't returned. If it returned I would have insisted on amputation. I suspect that Pegasus would still be here had I done that, but I did the best I could with what I knew at the time.
The tumor is almost certainly cancerous, considering its rapid growth. The longer you delay, the more likely that it will spread to heart, lungs, etc. When that happens, it is too late,
Doing a bunch of tests just adds delay. Tests results aren't going to tell you or the vet what is really going on. The fact is you and they still don't know. The only way to know is to operate, see if the bone is invaded, then remove either the tumor or amputate, whichever is then obviously indicated.
Good luck with whatever you decide.
bob