Depending on the frequencies involvd you are looking for either a "Frequency Counter" or a "Spectrum Analyzer."
"Inexpensive" is a relative term here, depending again on the bands of frequencies you are looking for and other functions you are looking for. What's your budget?
For lower frequencies (say, up to 1.2 GHz), you may be able to use a standard scanner. AOR, Yeasu, Kenwood, and ICOM all have units with a spectrum display.
Check out www.aesham.com, they have the above receivers as well as frequency counters (up to 1.2GHz for sure, maybe higher).
If you are looking or spectrum analyzers for 2.4GHz and above, you're probably in the many-thousands-of-dollars range (again, depending on resolution and reporting, features, etc).
Please state what you are looking to accomplish and perhaps a more specific recommendation can be made.
Good Luck
Scott
(Chicago)
