I think that the idea of a % split is quite good in some circs eg:

A performer or band is well known ish, but not on the front page of every mag all the time. They are looking to raise their profile out of their normal area (geographically or venue type). The promoter is tempted but not convinced. The performers say - well - we would normally get £x but will give you a first visit for a guarantee of £y against a % split of a/b.

The promoter isn't taking too big a risk, but the performer, confident of their fan base, should do ok too - the bigger their fan base, the better they do. It also encourages the performers to be sure to do what they can themselves in terms of promoting etc. The hard work does not end with securing the gig. Whether they do it themselves or pay an agent to do it, getting the gig is just the start of it.