The problem, however, is that no matter how detailed or well-written your spec is, it's very likely that it'll change, either as a result of business or product

The problem, however, is that no matter how detailed or well-written your spec is, it's very likely that it'll change, either as a result of business or product needs. This is especially common with startups. The best compromise that I've seen is to charge hourly, but make a solid estimate that you can stick to. If the specs change, you don't need to go back to the client or ask for permission to proceed. You just do the work.