Moqups is free while in Beta. Additionally, the main use cases will always be free (with some reasonable limitations).
The premium features will revolve around collaboration, feedback, annotations and other things. When these are launched, we'll definitely make the pricing clear and visible for everyone.
Good to know - place this information somewhere prominently on the landing page now! :)
I guess you haven't completely decided how your pricing is going to be, so I understand why you haven't published it yet, but seriously, you have buzz now, and people may not register today because they are unsure about the implications of it. Do not let that happen - give them at least the information you just posted.
Edit: It doesn't have to be super-prominent, but so that the missing information won't be an obstacle to registration.
1. Looks fantastic!
2. Where's the pricing? Coming from the landing page, I could not find it. You should really make this easy to spot.