If any UK FTOs eventually take up the MPL there is no way it will be cheaper that an Intergrated ATPL. With so much of the training required to be undertaken on a type-specific simulator this is not cheap. Compare £400-500 hr for an A320 sim compared with £250-300 hour for a twin. Possibly cheaper lower fidelity sims may be allowed to be used for some of the training but not all.
In fact it is possible that the MPL will be MORE expensive because of this.