how much would it cost to revarnish my guitar?

More details. You did mean the top, right? Acoustic or solid body?

Most likely more than it is worth. Unless it is a top brand as a Martin, Taylor, Collings, Gibson or Guild that is already worth into the thousands. Either acoustic or solid body too.

A solid body would be far easier (depending upon the neck assembly). One could take it to an auto body refinishing service for that 'custom' look.

I'd have to know what the make is. But besides all that, it would be difficult to just refinish the 'front' and not having to match up the new finish to the rest of the body to look correct, clean, and as new.

Hence, the expense to have it done by anyone in the business, or wood workers who have the experience.