I started a month ago and was thinking about the cost the other day too!
I totted it up:
Hybrid Bike £400
Clothing and helmet £150 (two days worth)
Some basics: Lights, Bottle, Cheap Lock*, Multi Tool: £50
Cleaning stuff: £20
That's £620 and I don't have: Puncture Repair, any pump whatsoever, spare tubes or anything to store this stuff! Priority wise these are less because I don't ride more than 20 miles, and that's no more than about 5-6 miles from my home which is 1) walkable if I must but 2) I have people who can pick me up. I even pass a
Halfords on my route if necessary.
That's 50% of the bike's price on additional things, and I haven't included my £89 GPS computer!
*I am not saying people should buy a cheap lock: I don't leave my bike unattended and I use this lock just as a deterrent while it's at home in case anybody thinks they can quickly break in and ride off with it if the nosey through the window. I am even considering upgrading to a Silver/Gold standard lock anyway.
Regarding 11mph, I am half your age and have been doing this for a month and I have only just started to average consistently at 11.4!