Risers = $18/ for 3
3 470's = $540
Mobo + CPU ~$100
32GB SSD = $25
650W EVGA = $70
OS: Free, if you pay for this... 
4GB RAM = $15
Moded BIOS: Usually free.
Case: $35 if you have the tools, $80 if you pay someone.
Total $803 USD or 776 euro's.