Dynamic Price Variable: Per Word!!

Hi!

I’m running a wordpress multisite, and need some help.
I’m intending to charge my users to post onto the blog. This isn’t new and not so difficult, especially if i follow this tutorial as I can then link it with paypal.

However, i want the cost per submission to be based on the amount of words posted. So for example, Ten Words = 10 Cents/pence. or 100 words to a dollar/pound. The currency doesn’t matter.

How can i write the php etc to by dynamic, grab the price based on the word count and then fit it into the above tutorial?

Please help!!
I’m new to this, but willing to do the leg work required!
Thanks!!
Sam