Both comments are entirely fair. The service/site are still operating in Beta, so the front-facing information is purposefully vague. However, you can get a much better idea about the site if you signup and work your way through the back-end. There is also a separate FAQ for registered users.
That said, to directly answer your questions, what will 50 credits get you?
1. it could buy you premium templates for 50 domains one month or 1 domain for 50 months (most premium templates are available for 1/credit per month). Examples of a premium business directory template:
BostonGolfing.com
AuroraCPAs.com
CapeBretonHotels.com
AthensAirportLimo.com
AstoriaPizza.com
AuburnHillsRestaurants.com
Use of free templates, like those seen at
http://www.cityboard.org/,
http://www.itexted.com/, or
http://www.spots.org cost no credits.
2. It could buy you almost two months of unlimited premium templates for up to 100 domains (see this link for all upgrade packages:
http://www.prebuilt.com/manage/#MTE=/Upgrade/LnRhYi1uYXZiYXItY29udGVudHx1bmRlZmluZWQ=)
3. It could buy you five months of an upgraded domain management package of 500 domains.
Signing up is free, again, so my recommendation is to sign up, add a few domains (also for free), and play around with the site a bit. Feel free to keep the feedback coming...we take all recommendations and suggestions seriously.