Credit based link indexer?

I currently use an indexing service with a monthly subscription, but I barely use it enough to make it worth the cost. Is there any indexing service out there that works on a credit basis where I could buy 10000 credits, say, and have them sit in my account for as and when I need them?

I'm just fed up of paying monthly for something that I am not getting the value out of.

Thanks

There is no such thing yet.
I am also looking for that.