I would only pay that size of deposit if I could pay on credit card and obtain the security that way, otherwise IMHO the risk is too big. Personally I would rather say you will book the work and say you will take the risk that prices of materials might change. When he actually comes to do the job and buy the material, he has the option to re-price the work and possibly lose the job and you have the option to agree or refuse. If he is busy, it doesn't matter either way because if you decide not to go ahead, the next job just gets pulled forward. In the commercial world, risks of large advanced payments are usually bonded or secured.