Fairly obvious at least to me that it is not possible to build an rc unit for less that what the purchase/ delivery price already is.
A Flysky FS-i6 with an i6 "B" receiver can be had for around Aus$65 and has been tested in open country to at least 2klm.
A good set of gimbals will set you back that much before you even start on the rest.
The B series receiver includes iBus as well as standard servo control and there are changes that can make the transmitter, 6 into 10 channels.