sh*t sh*t sh*t :( how much to get that repaired properly?
I've got 2 spots of rust next to the rear passengers side wheel arch. I was quoted £200 to strip the entire passengers underside panel (bit below the protector strip), respray it and with a 2yr warranty from JRM Edinburgh. Nowhere else seemed willing to offer a warranty on rust repairs. Doing the entire underside panel would ensure a spot-on colour match. Thus - won't be any more for your repair, hopefully less.
Because you're rust is exposed (mine is just bubbles), could you get away with painting it with a kurust pen and using touch-up paint over the top? I've just done that to a chip above my windscreen.