Well, if you are booking an act for only twenty minutes and possibly others on the same evening, I'd imagine that you wouldn't want to spend too much money on each one. Unless it's a very lucrative club, of course, but even then if you are paying out a decent sum you would surely want your money's worth in terms of performance time?

So, to a certain extent, you get what you pay for in these situations.

That's not to say all the acts will be bad nor should they be. It's possible that some will be very good "up and coming" musicians who are happy enough to do a short spot to help promote themselves.
You may also get good local experienced musicians who are happy enough to do a shorter set for the appropriate remuneration because the may be at a "loose end" and they may also be "friends" and supporters of the folk club scene.
On the other hand, there will also be those acts who are still "honing" their craft, "past their best", or just not up to it.