History:
Don't know much here I'm afraid, apart from it was used as a form of identification to stop students visiting houses of ill repute and mixing with the locals in taverns (at some point it became obvious students didn't need to wear large brightly coloured garments to avoid locals. Been to the Keys lately? How about the Stables?). Apparently up until recently three or more students wearing gowns constituted a procession and could legally stop traffic.
When to wear it:
Can be worn to debates. Definitely do not wear it while buying sundries in a supermarket - you just look like the sad case you probably are. Best advice is to wear it at night when there are fewer chances be seen looking like a tosspot, and it's warm.
Cost:
£110-130 new. Various used.
Size:
It has sizes from small to extra large. I'm 6"2 and weigh around 13 stone and a large suited me fine.
Anything else:
Conventions for wearing it are somewhere in the University's material. First years wear it normally, don't hook up the front (I don't know why - perhaps so you can escape from it more easily if someone normal sees you). Some people get strings in first year to hang off it. Second years wear it off one shoulder, 3rd years off other shoulder (left/right?). 4th years are called magistrands and wear it off both shoulders which makes it even more of a pain in the arse.
Tower costs money and you need to buy the chips for entry from the cathedral gift shop or as part of a multi-ticket from the castle. Castle on its own is free.
More info here:
http://www.thesinner.net/guide/Gowns
The Sinner.
"Apologies in advance for pedantry."