Cheap tactics for an open-source project

This is a bit disingenuous

If the ‘extra functionality’ is open-source as well, why don’t you make it accessible from the get-go?

And it seems the ‘extra functionality’ includes core features as well (Private projects, API).

This is a really cheap way of running an “open-source” project!

ClearFlask is Open-Source as per the official OSI definition.

Our prices are already low and barely cover hosting and support. If the benefit you get from ClearFlask is less than the prices we are asking for, likely there are more suitable alternatives.

If your budget is really tight, you are free to remove the licensing from ClearFlask (it's easy) and use it without financial contributions.