Cheap - M-Audio Flying Cow. WAY cheap, and pretty decent.
Inexpensive - ST Audio stuff. It's okay.
Reasonable - Here's where you get into the "real" stuff... Lucid, Mytek, Apogee, RME, Benchmark... More expensive, but this fits the "Good for the buck" category best. All are good performers and easily fit into a professional rig.
After that, you get the really cool stuff... UAD, the higer end Apogee, Lavry (my personal favorite), Prism, etc. This is where the point of diminishing returns occurs - Every penny of better sound costs an additional ten bucks. Worth it for some, almost a waste for others.