Could you be more specific about what you consider inexpensive? A Sytek is considered a good inexpensive mic pre for $800 or so. But around that price, you could also buy four single channel pres like the VTB-1.
Whether it's better to buy a mixer depends on how much you need a mixer as well as preamps. A mixer usually would include multiple routing options (including inserts for interfacing with outboard processors) EQ, headphone preamp/jack, and panning. If you really need those functions right now, then a mixer might be better. But chances are a dedicated set of four preamps for the same price as a complete mixer would have better quality preamps (although that's not always true).