The least expensive option I can think of is the SiliconDust HDHR3-US. I see it advertised on Amazon for roughly $84. To use it, you'll need to have a PC and an available Ethernet port on your router/switch.
http://forum.tvfool.com/showthread.php?t=820
When you install the configuration utility provided with the devise, you are able to tune to any channel and monitor signal strength and quality in real time.
The devise also allows you to watch and/or record TV on your PC. It has two independent tuners built in, so you can share the devise with other PCs on your home network.