You can always prototype in the Sandbox and use the 'dotcloud info' function to get current memory usage, so that gets you all the data before you ever push to 'live'.
Once you push a configuration to a 'live' flavor then you can also get an estimated bill via 'dotcloud info'.
Once you push a configuration to a 'live' flavor then you can also get an estimated bill via 'dotcloud info'.
There are some sample prices from real apps available too: https://www.dotcloud.com/pricing.html#slider_pricing