Rate of relay drops is likely around .1% or so, since FC seems to enjoy using values at such a low level.
Saying "3 in 30 minutes" also does not say much. The loot is not dependant on the time you are there, rather the speed in which you kill mobs. If you take a doctor into s10 you will certainly not expect to kill as often as an MA.
It's not actually hard to make an appropriate drop rate, but there is a variable you have to check as well. First, see which mobs are actually dropping the relays, and determine whether the stronger "type 2" mobs are dropping them more often or even the only mobs dropping them. While doing that, you simply have to count how many aliens you are killing (easily done by using your invader kill count) and then compare that to how many relays you have. A good number of kills would be 1000, but 2000 should be relatively safe if every mob can drop relays at an equal rate.
Number of relays / number of kills ~ droprate
Simple enough.