[zenpaul]

Good point. From this thread it is clear that people have made a go of it with project billing in some domains. I develop enterprise business systems that are generally too large and complex to reliably estimate or manage to a fixed budget.

In any case, all it takes is one project with one unreasonable customer who demands extra work and then doesn't pay, to wipe out the gains from lucrative projects.

Also, in the long term you accumulate responsibilities like mortgages and families that makes that low-risk monthly revenue stream so important.