There are a few things to consider before you contribute something:

Sufficient quality
Is the work you contribute of sufficient quality, or will someone else soon have to throw away all the code and redesign everything from scratch? If the quality of your work is not sufficient, its value might be zero, even if you spent hours and hours to do it. The question to ask in the case where the quality is not sufficient, is whether your work will help other developers implement a solution of sufficient quality. For proof-of-concepts, the quality of the work does not need to be high, because the purpose of the project is just to show that a solution path is possible.

On the other hand, if you say that you will complete a critical task, to replace all references to some insecure code, and you say that the task has been completed. If it later turns out that you missed a reference, then the value of the work you did should be considered negative, because it delayed the organization in getting the critical problem fixed and the next contributor has to start from scratch because he/she cannot trust the work you did. Profit Points might get deducted in such cases.

This last example means that there are very different standards for what is considered sufficient quality.

Is your contribution too big?
Let's say that the Paid Q&A project decides that a payment module is needed. You take the task and end up programming a complete service to compete with PayPal, then the community might either redefine itself to become a PayPal competitor with a small Paid Q&A service, or say that your contribution was simply too big to be accepted by the project, and therefore cannot be paid for. You are, though, welcome to take your contribution and start up a new project on the site. We do, however, expect there to be process for setting up new projects, to avoid the empty restaurant syndrome.