In reply to Alan M: Dakine do a split roller that's 100L and is cheaper but it's quite heavy if I remember rightly.
Or the osprey meridian is just over 70L and is a £100.00 cheaper than the rolling thunder, it has the wheels, and the shoulder straps, and a small daysack attached, carny remember the weight of it though, osprey and Lowe alpine have a few, take a look on the Ellis brigham website they have a fair few in their range at the moment,
Hope that helps,
Adam