Rafael,
I don't know the cheapest ones, but I can tell what they compromise relative to the high end product:
Spectrum availability - high end products have more frequency bands available
Transmitter output circuitry - high end units have better (and sometimes stronger) output circuitry that introduces less spurious noise into adjacent frequency bands, and is less susceptible to IM distortion
Compander quality - high end products have improved compander circuitry that are more audibly transparent
Stereo separation - high end products have better stereo separation and frequency response that extends closer to the 19kHz stereo carrier center frequency.
Receiver pack flexibility/durability - the higher end product have smaller, better built receiver packs with easier selection of frequency and useful features for the monitor engineer like frequency scrolling
Antenna combiner quality - high end product have better antenna combiners available for using multiple systems together.
Antenna options - high end systems have different antenna options for maximum flexibility in pattern control and polarization flexibility.
In ear amp quality - the high end product have better output drivers work with low input impedance multi-driver in ear monitors.
I suggest you look around for closeout and/or used Sennheiser G2 product. It is very well proven on tour, and has been largely replaced at the high end of the industry with the G3 product and/or Shure PSM900.
If you are outside the United States, then look around for US product in the 700MHz band that people are selling here as a result of the rule changes from the FCC in 2010.