If you buy some Bitcoins for $10, you have a $10 tax basis in those Bitcoins. Unless some subsequent event changes the tax basis, when you sell them or lose them, your tax basis is still $10. If you sell them for $50, you have a taxable gain of $40. If you lose them, you have a taxable loss of $10.
http://www.irs.gov/taxtopics/tc703.html
See also Topic 515 on theft losses. "The amount of your theft loss is generally the adjusted basis of your property because the fair market value of your property immediately after the theft is considered to be zero."
Of course, there's a form:
http://www.irs.gov/pub/irs-pdf/f4684.pdf
Ah, I see, thanks for clarifying. I appreciate it.
It sounds like you're trying to get the IRS to fully compensate you for your loss. This is tax fraud and you are likely to go to jail if you are caught. Do not claim a larger deduction than you are legally entitled to.
I did read what JoelKatz had to say. I just didn't comprehend it very well, obviously.