Frequency Selective Voltmeters
There are two non-commercial product options. One is that you go with something like this:



That's a MSP430 Launchpad with a Nokia LCD, but all of the signal processing is done on the MSGEQ7, a graphic equalizer display, commonly referred as a "spectrum analyzer".


  MSGEQ7- The seven band graphic equalizer IC is a CMOS chip that divides the audio spectrum into seven bands. 63Hz, 160Hz, 400Hz, 1kHz, 2.5kHz, 6.25kHz and 16kHz.


There are a bunch of different projects done with it, some have led matrixes, others have leds that vary brightness instead. Arduino too. There is a 5 band version MSGEQ5A (Frequencies between the 7 band version).

Another option is determining the frequencies yourself, through a FFT process using the analog in of any common microcontroller. Especially if is a small, known amount of fixed frequencies. WAY cheaper than any commercial spectrum analyzer, but it does depend on the frequencies themselves. Your microcontroller (and code) needs to be able to sample the signal at atleast twice as fast as the desired frequency (A mcu running at 1mhz will not accurately sample a 1mhz signal, even before you get into code slowing that down)

Edit:
You can even go old school discrete analog solution, by using bandpass filters to isolate the frequencies you want to measure, and measuring them individually.