I just wonder why you are missleading people, stating wrong things. "a couple posts per week " ???
The truth is this:
And this mean 6-7 posts/comments PER DAY.
Yes, 15SP is not huge, but it just feeds the basic needs right.
20-25 SP would be enough for super active, experienced newbie.
And this is half of your stated minimum of 50. And half of the money named.