bicyclewheelwarehouse.com sometimes clearances things for $100 or less, but cheap yet worthwhile rim brake wheelsets are getting harder and harder to find.
I'd just run the ones on the 3700 into the ground. It may or may not be worth maintaining the hubs - it's a bit of a losing battle with loose ball hubs if the seals suck, but if Trek bought hubs with decent seals, you should be able to get many years out of them. You can save a little money replacing the rims yourself, especially if you match the spoke bed diameter (ERD) to the previous rims, so you don't need to buy new spokes.
Outside of clearance sales, it's hard to hit a set of aftermarket wheels worth buying until about $200. See the MSRP on the ones on Jenson - they're about the bottom-level wheel I'd buy on purpose for a bike I rode for fun.
"Don't buy upgrades; ride up grades." -Eddy Merckx