Pricing Structure Question

Can someone clarify the pricing structure for PBWiki? I have a small business, and I would like two people to have full read / write permissions on the wiki. I would like the front page to be public, but i would like to give customers page-by-page access permissions, with the ability to comment, but not write as such. So the invited customers would need to register, but i only want them to be able to comment on a page i give them permission to view, not to edit.

From the information I found, i am having problems working out if I have to pay, and if so, what!

Thanks,
Callie