Ninja,
The Sauna Mania's Nothing Night on Fridays is only for this month or it will continue?. I cannot see any news on its website other than its special event of 1st annivesary on 11/11.
Their regular rates seem inexpensive - 100 baht on Mon - Tue, 120 baht Wed - Thur, and 130 baht on Fri - Sun.
http://www.thailandout.com/mania