Pay to post / buy credits to post

Hello newbie here, proud to join you all. Gush over, I have a question...

I am stuck somewhere [hopeless me] between Pay to Blog and Upgrades! I am trying to achieve a:

"buy 'credits' [via PayPal] that allow you to post, one credit = one post"...

Site Admin functions allow setting of price per credit with 'bundle' discounts - e.g. 1 post = $99, 3 posts = $249, etc... also need to set specific pricing per individual blog in my Mu installation.

I doubt it's out there all beautifully giftwrapped for me but if anyone has a direction they'd like to prod me in, I would be so grateful.

Also I don't really know the etiquette of commissioning/where to start (best place) to get this built if there's nothing similar I could play about with.

Was hoping for something nearer my needs in Premium plugins but typical me, I'm not a Pay to Blog or Upgrades kinda gal!!

Wishing you all a great weekend.

Jilly