I'm not saying that Heap have got the pricing right but you should absolutely NOT look at cost when you put pricing together - rather, you need to ask your customers about the value of what it is you're providing. For example, Heap will be saving the time and hassle of deploying a generic solution on a server and the time of customising it. Plus Heap will be improving their product every day whereas the self-hosted version would need a developer to add new features. These sort of things all add up - you'll be surprised what you find out when you ask your customers.

Pricing is hard. There are a lot of good articles and discussions on HN if you do a bit of searching.