Its up to you to decide really - you have to gauge what you think your members will be willing to pay.
I charge ?2 / $3.50 per month on my site, and offer discounts for those purchasing longer subscriptions.
It also might be a good idea to include some extra features for your subscribers, it will go a long way into tempting them into buying. To give you some ideas you can see what extras I offer to my subscribers here:
http://www.inbeforethelock.co.uk/for...us-member.html