I have one of
these [Kata DR-465i] and it's pretty good - enough space for a camera and 70-210 lens in the bottom (just) and a couple of primes + a jacket in the top and spare battery, sd cards, cable remote, lens pen etc
- it was £36 when I bought it last year though so might be worth shopping around or buying used. With a smaller prime lens on the camera I can fit the camera + lens and two other prime lenses all in the bottom compartment.
Depends what size tripod though - my Slik Sprint Pro is fairly small and light and fits fine on the side but not sure anything much bigger and heavier would?
Review of the bag here:
http://gudphoto.com/2011/02/17/bag-review-kata-dr-465i-camera-backpack/