I've done informal A/B comparisons between Pro Tools and 2 mixers: 1 a lower-end Alesis Studio 24 and the other a nice Soundtracs Solo Midi 8 bus. The most I can get with my system is 16 outputs, so I had to do some submixing- typically by just assigning multiple tracks to the same output.
For the A/B tests that I did, I did all of the mixing and automation in Pro Tools and set the faders on the mixers at unity. I then recreated the pan settings on the mixer.
Even though the Alesis, the mixs had a wider stereo field. It was pretty obvious that it sounded different, but it would be subjective to call it "better." I liked it better but I couldn't really describe why. It sounded meatier for some reason.
It a whole new ball game when using the analog EQ's! Man do they sound nice compared to the digital ones. The plugs are great for surgical tasks, but you can go crazy with the analog ones and it will still sound good.
Over all, mixing is easier with analog. It just sound good quicker to my ears. A little automation in the DAW and a little EQ from teh board and I feel like I have the best of both worlds.
As for how much to spend- seems like any decent mixer will add to your palette of options. I'd get one that has at least as many inputs as your DAW has outputs.
Take care,
Chris
I'd love to hear what you come with. Or possibly post the A/B comparison. I'm also very interested (this may be off topic) in A/B comparisons between digital software summing. 
