How about $25, $20 of which is a damage deposit that gets refunded after an inspection of the facilities deems that they have not been vandalized in any way. Make the process as demeaning as possible - a form to fill out, perhaps a waiting period before their refund, comments for the compliant "My aren't you tidy!" and public shaming for the slobs. Imagine the look on someone face as they walk away out $25 to chants of "Look at that guy wearing fleece/He can't aim it when he pees!" or other such creative stuff.
Then you can truly say the toilets are for everyone!*
*subject to certain terms and conditions.