Rick's revised and just out 2009 Europe Through the Back Door has a surprising amount of new material in certain sections. There's a section on relative prices in Europe's top cities, one price category is "cheapest sleepable hostel bunk" for 16 cities (prices are in 2008 dollars at exchange rate of €1.00 = $1.50): Paris $33/personRome $30/personLondon $40/personBudapest $20/person12 more cities in the book.
not to put a damper on the hostel thing (because they really are fantastic here!), but please note this is in regard to CITY hostels only, not hostels found in small towns.
I have been recently warned about a bedbug problem that has been spotted in many hostels in large European cities. This can happen to any establishment, but hostels are especially prone, so please inform yourself about how to spot the offenders and what your customer rights are.
Of course this can also occur in smaller towns, and this post isn't to suggest that every hostel is running rampant in critters. I only write this because several university students have told me that they and their traveling companions (separate groups, separate trips) in the past few months have had issues and heard of others having issues as well. Of course this is during the peak summer travel months, and when you'd most likely expect such a problem.
Think of this more of a PSA - just check as much as possible before you lie down, they can be difficult to be rid of!
I'm glad RS is adding more info on hostels in the books. I travel alone so hosteling is usually much cheaper for me in the larger cities.
The London price looks good to me, but RS should really check out the AIJ in Paris. It is 17 euros a night/peak season or about $25/night per bed. It is definately a RS kind of place - a clean place to sleep with a bed - no frills at all but definately worth the price and it has a great location IMO. I haven't stayed in the other cities you listed.