Can you file as a tax deduction the deposit lost on a cancelled contract with a homebuilder?

The best you could hope for would be that you could capitalize the lost deposit as part of the cost of acquiring whatever new home you did purchase/build. If you decided that it was not worth the risk/cost of buying/building a new home, then the lost deposit it just that - lost, with no deductibility. It could, however, be rationalized as part of the cost (basis) of whatever home you did acquire (presuming you did). Otherwise, it's as deductible as a lease-purchase option should you decide not to exercise the purchase option - not deductible. Sorry.