as alternative of the Asrock http://amzn.to/2oJm6ua bro btc 1 or http://amzn.to/2oqj4aUpro btc 2
found some of lower price 6 gpu and 7 gpu motherboards

7 gpu motherboards :

• MSI Z170A Gaming M5 http://amzn.to/2pz7egn  130$
• MSI Z170A Gaming Pro Carbon http://amzn.to/2pzhcP0  140 $
• Gigabyte  GA-Z170X-GAMING 5 http://amzn.to/2peJNvN  140$
• ASRock Z170 OC FORMULA http://amzn.to/2oSXqzX  233$

6 gpu motherboards

• MSI Pro Z170A SLI http://amzn.to/2oqlYMQ 110$
• ASUS H170 PRO GAMING http://amzn.to/2oSXNdN  110.5 $
• MSI Z170A SLI Plus http://amzn.to/2oT3rg9  115$
• ASRock FATAL1TY Z170 PROFESSIONAL http://amzn.to/2pzl3vs 135$
• ASRock  Z170 EXTREME4 http://amzn.to/2oJsTUB  138.5$
• GIGABYTE GA-Z170X-Gaming 3 http://amzn.to/2o1lIbV  165$

sure there is more motherboards but need more search time
if you have any of those please give a feedback