I actually wrote a blog post about milongas for none dancing tourists to go to.
http://www.therealar...op-10-milongas/
I think if you aren't too loud, don't walk on the floor ( or "try" to dance) or take photos with your flash on. Most dancers won't mind you being in the milonga. They actually probably won't notice you at all.
Otherwise someone just told me of small tango show that is only 150 or so PESOS and was relatively good.. no dinner etc... It's on corrientes I will have to find the address.
If you need more help let me know. I am starting a small business in tango tours for non dancers.
Good luck!