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Hello, this is Dr. Bruce and I would like to help you with Emma.
That does seem like a lot of money to do a muscle biopsy. I think that it would be a very good idea to get a second opinion.
I do agree that getting a biopsy on a lump is a good idea, but I don't know how that price has come about. I am typically able to do a needle biopsy for less than $200. Although the needle biopsy is not going to be entirely diagnostic in all cases, it is a good first test to do to screen for something that may or may not be serious.
As for whether this is myositis, it doesn't sound like it to me. Usually there is pain opening the mouth, and possible muscle atrophy. See this article http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=1755
Since you indicate that Emma has no other symptoms, it makes me less suspicious that masticatory myositis is a real concern.
I would certainly start with the needle biopsy, see what you get, and then make a decision as to whether this lump needs to be removed or can just be monitored.
I hope this helps and that Emma does well. If you would like to continue this chat, please reply to this post and I will get back to you as soon as possible.