Why are they so expensive? Tbh I don't really know. I can understand that with more money comes a better quality of accommodation but I really think they're built for conference guests rather than the students, and the price goes up each year beyond the rate of inflation. (Holland Hall was £120 a week when I started in 2004; it's now £147 and you don't even get lunch during the week or any food provided at the weekends.) If you ask me Exeter is just being greedy and money-grabby.
The most expensive rooms (at £140 ish) are catered and ensuite, though I stayed one night in a room in Holland Hall and found the bed uncomfortable and the whole thing very Travelodge-ish. (examples: Holland Hall, Birks Grange, Pennsylvania Court)
Catered non-ensuite rooms are I believe in the region of £110 a week (possibly slightly less). (examples: Lopes, Hope Hall) Having seen the rooms I think they're very nice and of a good size, even the single rooms that you don't have to share with a roommate.
Self-catered ensuite are £95-£100 I think. Having stayed in this for a year I'd say it's alright, though there were a lot of things that went wrong with the electricity and plumbing during my stay (have also heard reports of a similar ilk from students in other buildings of this type of accommodation). (Examples: St Germans, Rowe House)
Self-catered non-ensuite is about £65-£70 a week - having lived in this for a year I loved it and would not hesitate to go back. It's the best value of the lot and imo the most fun too. (examples: Cook Mews, Lafrowda, St Davids)*
*let us know if you have questions on these - Sephrenia lives in Cook Mews, I lived in Lafrowda for a year and my boyfriend used to live in St Davids!