Urban airship were interviewed on the Mobile Orchard podcast a few weeks ago, so take a look in the podcasting section of iTunes to have a listen to them and find out what they offer.
They charge $29/month for 50,000 pushes. Above 50K they charge 0.1 of a cent per message. They do have an unlimited push plan for the big pushers out there. These guys are ones that Tapulous turned to when they decided to add push.
They give 25,000 free messages/month ( mpm ) and then charge .05 of a cent per message ( half the price of urban airship. )
When you see those numbers it looks pretty cheap. 25k pushes for free etc. But at .1 of cent if you charge, say, 99 cent for a pushes add-on you could only send ~700 pushes before you start to loose money with one of these services.