As an ex-developer for the DCS combat system I will have to say a set price isn't always the answer. For example, I generally do not work on scripts which can be taken from the script library, so things like door scripts, simple vehicle scripts, etc are things I don't do, because for me to take on a job, I need to have least some interest in the project itself. Also depending on the scope of the project and how widely I think my script will be re-used, such as a general purpose melee weapon script with functionality built in so that the item creator only needs to use keywords in each prim's description to specify parts which are to be handled by a color menu or draw/sheath functionality or particle emitters, I would ask for a percentage based on sales of products which use that script. If the script I write you can be used in multiple products, rather than charge you a bundle up front, I'm willing to take percentage on sales. This of course would come with some contractual agreement because there is a trust issue that the content creator will make sure to give you a sales percentage on each of his or her items which use the script.
This kind of situation seems to be more what I prefer as now I have an invested interest for your product to sell well which is good for you as then I am more open to making necessary changes. In the end, it's a limited partnership where both parties can benefit..
Typically the percentage is usually determined by the estimated retail price of the item and the complexity of the script.