What exactly are you finding (presuming new) for $200?This is a substantial research and discovery. I can find a transformer for 200$ whereas rotary phase converter (RPC) costs 2,5k$ .
so far I told customer a price for RPC and he replied ‘too expensive’
Will this aeropoane ‘fly’?
I'm thinking to build your own open delta out of two single phase transformers you probably needing at least 2 7.5 kVA minimum possibly bigger if I missed some the loads this is just for the two motors you mentioned in OP.
I think those alone probably going to cost you around $1500-2000, and you will get either 240 corner ground delta or 120/240 high leg delta as a secondary voltage - if that is something your equipment can take for input volts. But keep in mind a RPC will have two output lines that are the same as input volts but the third leg will not be a steady voltage either. The open delta will have significantly less operating and idle losses than a RPC.