A Great step in what I consider the right direction! I would like to suggest a change though.
After some testing it appears that the person is only getting the set number of ads free, then never drops down into the free level again. So, if you set the number to 2, the person gets 2 free ads then always has to pay for them.
I would like to suggest that it would be nice, if a user never has more than 2 ads running he will never have to pay. So when an ad is deleted or expires, it no longer counts against their total.
I also think that it would be nice to be able to have different settings for different ad catagories.
So I can allow anyone to have one free car for sale
ad running at any time, but I can also allow them to have 3 free parts for sale
ad running at any one time.