If you don't need 192 channels and very complex triggering, you can probably live with the USB. If it's fast enough for what you're doing, it will cover almost all of your requirements.
You can also find things like the HP 54620a I just picked up for $50. 16 channels and 500 MHz acquisition. No serial protocols, of course, and not expandable, but it covers a lot of smaller projects. I actually bought it to get the cable for my 54645d MSO.
I have a 16500b mainframe loaded out, but I don't bother hauling it out if I can make do with 8 or 16 channels. They are fun toys to play with and with their programmatic ability, there's not much of a limit to what you can do with them.