Plan for $2,000 to $5,000 for a properly planned mast cell removal with adequate margins, more where the location is difficult or reconstruction is needed, and a few hundred dollars beforehand for the needle test and staging that should precede any excision. A simple lumpectomy price on a known mast cell tumour is a quote for the wrong operation.
Mast cell tumours are the classic case for doing it right once: they extend microscopic fingers beyond what you can feel, so proper surgery takes a planned margin of healthy tissue. The pathology report afterwards, which grades the tumour and confirms whether margins are clean, is not an optional extra; it is the difference between finished and falsely finished.
Budget lines owners forget: the histopathology itself, a recheck schedule, and in higher grade tumours a possible oncology consult. If your quote is a single line that says lump removal, ask for the itemised version and where the margins are in it.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.