You won't be able to build a rig with a lot more than one or two GPUs if you have to buy all components!
For example:
Motherboard: 80$ (ASRock H81 Pro BTC R2.0)
CPU: 70$ (Intel Celeron G1840)
RAM: 60$ (Corsair Vengeance 8GB )
SSD: 60$
PSU: 160$
Risers: 50$
(Frame: ~50$ if you build it yourself)
_________________________
530$
With the remaining 370$ you can buy gpus like (2 x gtx 1060) or (1 x gtx 1070)!
have fun building
Excessively unnecessary parts and expenditure!
i can buy 60$ and 100$ around cpu+mot+ram
Riser 5 pcs : 15$
PSU 150$
Disk : 5$
Frame : maks 30$
Maximum $300 Bro