The band I'm in paid £80 for a recording of one of our gigs. Guy turned up with a laptop and firewire interface for 8 channels, took a line out of the mixer/patchbay for each channel, then 2 weeks later we received a mixed 30 min CD of our entire set, and it sounded great.
A few gigs later we were offered the chance to record again, but declined (it was our new bands 1st gig and we thought it could be a bit ropey!). On the night the sound engineer turned up as he was recording the headline band. He approached us and offered to do it for £40 rather than the usual £80 as he was there with all his equipment set up anyway. Obviously we were asked not to mention it to the headline band. Again we received a great recording!
Point I'm trying to make is that if you 'get in' with a few promoters and/or venues you can offer to record gigs for bands. The details of the particular sound guy were given out via email when we agreed to do the gig, so i'm guessing the promoter may be taking his %10%, but potentially you could make say £320 recording a for 4 band night... or at least £80... and obviously once you knew the acoustics of the room and the equipment etc a standard, half decent quick mix would take too long.
I actually prefer a couple of the live versions of our tunes to the originals simply because of the energy and the audience response.
Don't expect to retire, but it could certainly supplement an income if you have the equipment, and im pretty sure you wouldn't need public liabilty insurance etc (just have your gear covered... esp for theft!)
3 bands a week = £240... which (before tax) would give around that £12k figure.