Shreve Systems (http://www.shrevesystems.com)offers a IIvx to 7100/80 upgrade with motherboard trade-in for $250 ($300 if you keep the IIvx board), so it can be done. I have no idea if Shreve offers the best price on this and I've never dealt with them before, but it can be done.
That said, the previous poster is correct that getting a different machine would certainly be the better move financially. For instance, Shreve also sells a refurbished Power Computing clone of the 7500/100 (minus the A/V ports) for $100, though you'd be shelling out more to outfit it with memory. That would get you a clear path to a G3 upgrade if you so desired. If you're really attached to the idea of a 7100, they can be had on eBay for under $200 including memory, drives, etc. Or if you don't want to go with a clone, 7500's are going for around $300-400 on eBay.