No, mine are not "off", & your's are not either. They are just assumptions. Either of our assumptions cpuld be correct.
Here is why I am assuming $10-$11.
The inputs include percentage of members, price of monthly pass, and washes per member. I think 40-70% member washes is realistic. Monthly pass price averaging $30, and 3 to 3.5 washes per month per member.
70% members, $30 per month, 3.5 washes per member =594,000 OR 60% members, $30 per month and 3 washes per member = 672,000
One thing to add, this scenario makes it look like is it more profitable to have less members. That is because we are assuming identical volumes. In the real world, more members = higher volume so it would be highly unlikely the the same site could wash the exact same number of cars with or without members.
In my opinion, the key to a monthly pass is limiting it to one vehicle. Most (maybe all) of the operators that are unhappy with their monthly pass at an inbay, are using a system that cannot prevent multiple vehicles from using 1 pass.