Question to experience producers, I'm interested in the process of making remixes.

How the process work, lets say if someone wants to do a remix of some popular track do you need to:

1. Aquire the project file and therefore always contact the original producer and purchase the rights?
2. If the above is true, can the owner refuse? Common sense say yes but how to have more chances for success?
3. Anyway to predict the price of such a purchase?
4. Is it legal to do remix without original project file just with original purchased track?
5. Are there any examples of popular remixes done without a project file?
6. Are there other things to consider?

Thanks