I know a wireless guitar transmitter system works, but that's kind of expensive- does anyone know of a wireless transmitter set that works for metronome and speaker that's under $100?

Wireless technology that works is more expensive than that.

You could get an Audiotechnica ATR7000 Series VHF wireless system designed for a guitar and plug a metronome into that, but VHF wireless technology doesn't work very well. Plus, the receiver needs to be powered with 120VAC and uses a DC transformer, so you either need to get power to it or hack up your transformer cable and build your own battery pack. Price will likely be less than $150, plus whatever you materials you need to supply power. To give you a sense of how crappy this is, you can buy an Audiotechnica ATR7000 Series system at Staples. It might work for a just a click, though, because sound quality isn't important.

An Audio Technica Pro Series 5 will give you just about the cheapest UHF wireless system out there. This will cost a few hundred dollars.

When you account for the cost of a rechargable battery, loudspeaker, wireless receiver, and wireless transmitter than come with something like a MegaVox, you will find that things work out to about the same price.