I've never shipped wheels, but I have received them. The cost was around 80 dollars for two boxes of two rims, each package roughly 40 pounds. This was from NY to me in AL. It was shipped by UPS to me and arrived without problem.
UPS is the way to go if you're shipping large, heavy items like wheels. They will have cheaper rates usually than FedEx will. USPS is the best for smaller items, not as much for big large/heavy items, so I wouldn't look to them first with shipping wheels.
Like Brent said, get a scale and weigh them packed and ready to go and check rates.