It's not so easy to stay under $200 if you mean all parts apart the RC radio. I mean 4 motors + 4 ESC BLHEli32 with telemetry + FC + WIFI, compass, 1 down-facing rangefinder, .. in particular for the FC if you select these components with quality chips and sensors it starts to be quite challenging. Furthermore, if you would like to develop swarm applications the time spent to create the fleet is not to be underestimated. Last plus its a platform with 1 single common 18650 LI-Ion battery for higher flight time in comparison to the usual LiPo.

Yes, it depends on what kind of sensors you get. I meant just with GPS, 4 BLHeli ESCs will run you around $40, 4 motors another $40, Kakute F7 controller another $50, $30 for the frame, $8 for the GPS, $10 for the receiver, $5 if you want an extra PDB, it's not that bad.

If you add sensors it does add up though, you're right.