No, not necessarily. It depends on how often and badly the org has been burned on past project failures.
Typically, the intermediate step is to tie some compensation (such as a bonus) to a particular metric, but why not just go whole hog?
Even if X% is projected to be larger than $Z (and in fact, the better the project succeeds, the larger the %X total ends up being, but you can cap it, or make the % progressive up to a point and regressive above it), the advantage for both client and vendor being aligned on the upside is considerable.
Which still loses to the other bidders who say "$X in $Y months for $Z dollars".