Was at the same gig, but we missed most of Janes Addiction beccause my wife was getting a tattoo and it took longer than we had planned for - had to get a taxi from hornchurch to the O2, but luckily it was so late it was easy to get in and there werent any crowds, it all worked well, and enjoyed the gig. But yes, 3 and a bit hours there and back, but we stayed overnight so another £100 or so for a hotel, and the cost of the tattoo, and the shopping the following day, all in all the cost of the ticket was practically nothing!
But in general going to london is very expensive, so we haven't been in a long time now. We went up for Too in 22l, but that had been the first for a while and probably the last for a long time (apart from gatwick on the way to new york, but that doesn't really count).
There are no groups that I would pay over £100 to see and that seems almost normal for there now.