There's several things to consider:
Which OS are you on? some Interfaces work better on Win, others on Mac.
Generally speaking Firewire is better than USB (less load on the CPU) but also more expensive and most win laptops have bad firewire chipsets, so you'll just end up with a lot of problems.
Usually the price depends on the following qualities:
- how good the preamps and the converters are (read some reviews and user opinions to find out more about it)
- What resolutions and bitrares does it support (some go as far as 196K and 24bit) Depending on the work you will have to do with it you will need more or less...
- how many inputs and outpus it has (how many do you think you need/will need)
- if they have midi or not (do you need it?)
- if they have onboard DSP (most of them don't)
And there is some nice to have features depending on the use you will make of it:
- real Hi-Z inputs for e-guitars and basses
- zero latency monitor: input audio does get routed internally to the headphones without going through the computer, so you don't have any latency. This is useful if you plan on working with overdubs a lot.
- small and sturdy casing: some are made of cheap plastic and are quite big (like the E-mu 0404) some others have metal enclosures and are quite compact... again, depends on what you need to do with it.
This said there is few good usb 2.0 interfaces
E-mu 0404 (good preamps and converters for the class, has zero latency monitoring) costs about 200€ here in Europe (this is the one I own)
Tascam US144-MKII seems to be a nice interface, costs less than the E-mu but preamps are not as good
And there is several USB1.0 ones:
Focusrite Saffire 6 USB: is more for DJs than to make music has some average quality specs (not as good as the higher end Saffires)
Motu just released a new small usb1.0 interface called MicroBook. Motu makes some great audio hardware, but I don't know much about this one.
The newer Cakewalk/Roland interfaces seem to be quite good, albeit a bit pricey
just to name a few...