With a 100 active users at any one time? No it's not expensive since over the course of the month you would have hundreds of thousands of users. If you have 100 active users for your app you most likely could use the free/$100 option as it looks like it scales to requests per second which to be at 800 requests per second you would need a LOT of traffic. Highest I've worked with was 500 requests per second on a site handling 250,000 users a day and they only had that at peak times when they got 50% of their traffic in two hours.
I think you would need to look at your http logs to see how many request per second you're currently getting, I highly suspect it'll be nowhere near what you think it is.
Agreed. Our games are played by ~200k players a day and before we added fast-polling for live-play, our peak was around 500 requests/second. That's 65k new games a day and millions of moves - obviously not directly analogous but you get the point.
I looked at parse.com for a colleague and the pricing before you become enormous seemed quite reasonable.
I think you would need to look at your http logs to see how many request per second you're currently getting, I highly suspect it'll be nowhere near what you think it is.