There a similar service, call Pusher.
There are some differences. Technically, Pusher uses Websocket while PubNub uses Http Streaming/Long Polling techniques.
But both techniques work pretty well, so that aside, let's look at their pricing.
The difference in pricing is that Pusher is somehow simpler and better than PubNub.
Another the number of from messages Pusher looks to be more.
If I will to use PubNub, looks like the highest tier Galaxy plan is the only choice to go.