as alternative of the Asrock pro btc 2 https://goo.gl/MCg5in
found some of lower price 6 gpu and 7 gpu motherboards on Ebay

7 gpu motherboards :

• MSI Z170A Gaming M5 https://goo.gl/wqVVDd
• MSI Z170A Gaming Pro Carbon   https://goo.gl/ALu677
• Gigabyte  GA-Z170X-GAMING 5 https://goo.gl/cRSfNj
• ASRock Z170 OC FORMULA https://goo.gl/HEibpA

6 gpu motherboards

• MSI Pro Z170A SLI https://goo.gl/RUrJcL
• ASUS H170 PRO GAMING https://goo.gl/bBKaFK
• MSI Z170A SLI Plus https://goo.gl/PR5kZU
• ASRock FATAL1TY Z170 PROFESSIONAL https://goo.gl/w8p8Ha
• GIGABYTE GA-Z170X-Gaming 3 https://goo.gl/r3aYSv

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