G31 is a stripped-down G33... It has a 4gb memory cap (vs. 8gb on G33), does not have HDMI functions, and it also uses an older southbridge that offers fewer USB and SATA ports. There's also a bit of conflicting info whether G31 officially supports 1333fsb, but most G31 motherboards appear to have the option anyway, so it's really a non-issue.
TBH, when comparing just the chipsets themselves, there's not much functional difference for most people's purposes.
Most of the price difference you're seeing comes from additional features offered by specific motherboard manufacturers, beyond official chipset specs. Stuff like number of DIMM slots, additional SATA controllers/ports, upgraded onboard sound chips, etc. G31 boards are likely to be far more bare-bones than G33 boards, and more likely the latter are going to be the better value for your money, unless you only need the absolute barebones features.