The videos posted below show you how to add diversity to your current FPV setup but dont want to buy a whole new diversity receiver. It is limited to just 2 receivers but costs less than $2 in parts. You will still need to use two receivers (RC305 are recommended). The way the diversity controller works is it monitors the signal from your two receivers and switches to the one with the best signal. This is a great weekend project for anyone to try, but I do suggest that you practice abit of soldering beforehand (see the soldering basics video guide).
Diversity Controller Parts List
LM324 14-pin DIP package ($0.57)
CD4066 14 pin DIP package ($0.52)
1 x 820K or 1M ohm resistor between pins 3 and 1 of the LM324
1 x 0.1mfd capacitor from pin 3 of the LM324 to ground
1 x 0.1mfd capacitor from pin 2 of the LM324 to ground
The resistors can be purchased anywhere and can be any power rating from 1/8W through to 1/4W. I'd expect to pay no more than $0.05 each for these.