Zend PHP Engine is not an expensive solution.

Not applicable.

We use the solution's open-source version. Also, its paid version is not expensive. Its pricing depends on the number of users.

The solution is open source and you just pay for the server cost.

Zend PHP Engine is open source technology, so there's no need for us to worry about licensing or other costs. The only costs involved are indirect, i.e. what we have to pay for server hosting and related infrastructure services. 

No problem with pricing and licensing, as tools are open source. You have to consider training cost, mindset setup cost, as part of the TCO.