What is the best commercial model in outsourcing?

As a commercial model? It depends on the business area you are talking about. To program software, the most efficient way is to go through a fixed rate after agreeing on a very detailed BOQ. If it is about a contact centre, you should look into your historic numbers, patterns and all kinds of forecasts (calls, marketing, product launches, churn...). You can then calculate and decide what is best: productive hour, man hour, fixed call rate, fixed monthly rate, per agent/head rate. It goes without saying that a new business without historic numbers should never agree on a fixed monthly rate.