If the objective was to re-route the Hayes service then yes, surely much cheaper. Any tube extension surely has to tick a couple of boxes:

1. Improve service to poorly connected areas;
2. Relieve congestion, especially in the central area

Re-routing the Hayes line would do neither of these things but would instead annoy a lot of Hayes commuters who have fast links to the London Bridge/West End/City now. Unless you propose augmenting the existing service with a route via the Overground in which case this would probably exacerbate congestion at Lewisham.

AFAICT, the only reason Hayes is mooted as a southern destination is because, past Lewisham at least, the route is not crossed by any others so service frequency could be reliable to tube standard. The reason why the Bakerloo is ripe for extension is because it is the only deep tube line with spare capacity.