I realized today that when you use a spaceport to send population away to another planet, it lowers that number of pop on the Empire population screen until they reach the new planet. This means if you want to make it cheaper to boost a population, just do a mass reorganization of them around your systems.  

I'm wondering if this is an exploit. It doesn't seem particularly strong, but it's a good way to be able to afford a boost when you are low on a resource and don't have acces to the market.

It's not

It's not considered an exploit since while in transit, they're not actively producing anything for you(plus they can be killed en route by Hostile Actors... making it a risky action). As such the population booster only affects population who are actively producing on your planets. One can argue its an effective way to boost minor population without having to spend a lot of luxury resources to do so(since you mostly want it for propagation than actually boosting their yields) but outside of that, there really isn't any benefit other than boosting your own major population(which if you're looking to boost Minor Population growth is counter-productive since it also boosts Major Population propagation).