I'm paying my Ride to Work voucher off (bike, rack, lights, winter tights on sale, panniers and lock) at £25 a month.
I've spent about £40 on clothing and bought a new set of brake pads for £7. Two inner tubes and repair kit stuff at about £20. Plus silly girly accesories that I don't really need and a couple of professional check-ups a year.
So all together per month I'd say £40 a month for the bike. Plus say £15 a month on my oyster card for when the bikes in the shop, I'm going out to get a bit squiffy and can't cycle etc. So £55 a month on all my travel (I don't drive).
Sounds expensive until you factor in the fact that I used to spend £77 a month on my travelcard plus around £20 a month to get into zone 1, a grand total of £97 a month.