A really simple message passing system is http://aws.amazon.com/sqs/pricing/
first million free, then 50 cents per million after that. I suspect you will do something fancier and cost more...
Personally when I look at services like this, I look for 2 things.
1) Free tier. Not time based, but volume based. If some prototype takes me 3 months to write, I don't want to run out of a free demo 2 weeks in.
2) Does pricing scale in a sane way? Or does it go free... free... free... $5,000 a month! If I see an insane price scale, I will also avoid something. I always price something vs doing it myself.. and for a smaller app, I could never afford 5k so why even mess with the product.
