If you want to build a really cheap drone that can actually do autonomous GPS-guided stuff, the Kakute F7 can be flashed with Arducopter (Arudpilot), and is $49 with a power distribution board. Mostly intended for small racing drones, but can be used for other purposes.
https://www.getfpv.com/holybro-kakute-f7-aio-flight-controll...
https://shop.holybro.com/c/kakute-f7_0486
Or you can get the version that combines a four motor ESC in a stack with the version of the Kakute F7 that doesn't have a built in PDB (power distribution board), for under $120.
https://shop.holybro.com/kakute-f7-amp-tekko32-f3-metal-4in1...
other parts needed to complete something: motors, propellers, frame, battery, video tx (for something like 5.8 GHz band goggles), RC receiver, wiring, ublox m8n GPS receiver.
want a cheap frame? It'll be heavier than ordering a $45 carbon fiber frame from banggood or gearbest or aliexpress or such, but you can 3d print one at the cost of maybe half of a spool of $18 PLA filament: