One interesting approach I remember from a previous thread on this topic, was to charge for writing a spec, but discount it from the project total if the client

One interesting approach I remember from a previous thread on this topic, was to charge for writing a spec, but discount it from the project total if the client decided to go with you. That said, if the spec is large enough where you're going to be investing a significant amount of time into it, I don't see why it should be any less billable than the other work you do.