£50 for a student / non prefessional permit.
But hey, just do it. If they ask then you say you didnt know, very sorry, will delete the images and then move on.
Photogrpahy in a public place of anyone or anything is permitted. So you can stand on the opposite side of the strrt to a station and take a photo, but not in the station itself.
http://www.tfl.gov.uk/termsandconditions/14696.aspx
Rule 15 is a funny one.....
15. LUL does not permit filming or photography of the following activities:
unlicensed busking
begging
smoking or use of illegal drugs
vandalism or graffiti
ticket touting/fare evasion
assaults on passengers/staff
use of firearms or other weapons
activities of an overtly sexual or otherwise indecent nature (including nudity)
So althoughthe above goes on (particulary the first 5), everyone knows it goes on, the station / police etc know it goes on....you cant photograph it!!!
However, given the above, i have seen photographers taking snaps without a permit. Tourists do it all the time. I have done it (inadvertantly as i didnt know you needed a permit at the time)