Im always torn on this, on one hand the dep is getting them out of a whole and if the dep is good and the band still sound the same then part of me says the dep should get an equal split for being able to fill the slot, on the other hand the dep does not pay for their share of expensive items such as the PA and lighting rigs etc so maybe a small cut should be taken back for that?
In my own head then if the gig is £600 and you have saved the day and played well etc then I would be happy to take the full £150 if it was a 4 piece band but maybe still happy to do it for as little as £100-125 if it was local etc and a one off, if it was regular then them £50s will start to stack up and they are getting a bass player worth the extra if the gigs are still coming in.