One of my friends does a productized (fixed price) roadmapping session (1) as part of a larger "build your MPV" service. My understanding is that it works out really well for him and his clients. They get to try him out with a fix priced engagement. The output leads naturally to the next step in a larger engagement. If for whatever reason it doesn't workout the client has useful documentation they can take to someone else.
(1) https://www.reemer.com/consulting/build-mvp