[Xenon]

i also request that half is paid upfront

that's a general behaviour

I don't know actualy prices for style changes. But for coding works, it's not rere to pay per code line (if you have a good and efficient coder)
0.50$ per codeline are normal to my knowledge.

Some are also offering flat prizes, for example small hack 5$ ...

It also depends on the time after the work is done.
Is the author allowed to sell it later to other people as well, or do you want it exclusively?
You have to consider much things, so a general answer is hard to give.