Hello Photopost Classifieds natively does this
1) Per-ad cost membership, where users pay fees for each ad they post
Now I do know in the next edition of Classifieds we are looking to move the payment from a global option to a usergroup options which in itself is quite an easy modification to the current code should you wish to do so.
Basically to acheive number 2 below you would move payment to a usergroup option not global and the group you want monthly payment from you set payment option to no in usergroups for there usergroup after say they are moved by signing up in
vb subscriptions for a subscription service. I have done the aforementioned hack to a couple installs for customers and we are looking to make this change like I said in the next version. I have not documented the modification but if I get any free time I will try to do so
2) Per-month membership, where users pay a fixed amount each month and are allowed to post up to a maximum of, say, 3 ads in each category per day.