How much does it cost to build a rig??. It's all depends on hardware what are you choose. Like a motherboard, an SSD, a power supply, a processor, and GPU cards. Just for example:
1. a motherboard ( Asrock Pro BTC H81)
2. a Processor (Socket 1150)
3. SSD (64 GB or 128 GB) suggested Samsung.
4. RAM (DDR3: 8 GB)
5. PSU (adjust to the number of GPUs) can buy EVGA.
6. GPU (AMD: RX 480,580, Vega or NVIDIA: 1070,1080 cards).
7. PCIe USB Riser

for a price isn't stable.