The going rate varies a lot depending on things like size of the room, whether or not you're sharing it with someone else, the desirability of the location (good or bad part of town; good visibility for sign advertising; potential for referral from other practitioners, etc.) But the biggest influence on rent rates is where you live. For example rent in NYC is going to be a lot higher than rent in rural Iowa. If you want to see some examples, check out the "for rent" section of this forum:
http://www.massageplanet.com/forum/viewforum.php?f=39
I briefly scanned some of those postings and saw room rentals ranging from $230/month in Nacogdoches, TX to $750/month in San Ramon, CA.
Your best bet is to call other massage therapists in your area and ask what they're paying. Or, call the room rentals available in your area to get your local range. If you're not sure where rooms for rent might be, you can call your local chiropractors, acupuncturists, naturopaths, health clubs, physical therapists and other bodyworkers and ask if they have a room available. Good luck!