[rogerbinns]

I charge by time, even though I am producing results. The biggest advantage is that it deals with changes in the specs, addition or removal of features, new environmental issues (eg also test on ARM, use a cluster).

If you charge for results only, then any change in what is desired means a renegotiation. Attaching a dollar figure to every "could you also ..." gets old quickly.

For your regular employer what you are after is ROWE - results only work environment - https://en.wikipedia.org/wiki/ROWE

[JoeAltmaier]

A hybrid approach works fine too. A fixed price for the prototype on a date; time and materials for changes after that point.

[soft_dev_person]

Changes in the specs requires negotiation anyway if you have a contract with an upper time/cost value. Charging by the hour doesn't solve that, unless you have a no limit contract that puts more or less all the risk on the client.

This is very rare where I work.