Hmmm, a bit tougher than you might think to answer that. I buy a fair number of PDFs, and the amount I am willing to pay depends entirely on the product.
In general I buy things that fall into two categories:
Those that I will need to print multiple copies of - forms are a prime example, I have used Campaign Planner a fair amount for example, as well as Character archive. I bought both on DMs Day, and they were less than $5 apiece.
Terrain such as World Works is another example of this, and I pay around $15 for them.
The other category I like in regards to PDFs are the ones where I am not likely to need to print the whole thing at once. I have used both Book of Templates and Librum Equitus compiled in this manner. I paid around $7.00 each for these. if I recall.
Though I do own other PDFs that fall into neither category - A MAgical Midieval Society: Western Europe leaps to mind. I paid $10 for this, and found it well worth the money.
I would say that $5 - $7 would be a fair range, depending on the nature of the material.
And let the community know what you are selling! A sample that shows the quality of the product is a good idea - it was the free sample from MMS:WE on cities that sold me, if it hadn't been such a useful sample t might not have done the job, but it sold me not only on MMS:WE but on the sequel that covers cultures and world creation.
Oh, and by the way - I have found the script that you made for Fractal Mapper pretty handy, just thought that I'd take the opportunity to thank you in a public forum.
The Auld Grump
*EDIT* And folks should bear in mind that the sales on PDFs are small enough that the price really can't drop below a certain point - it just isn't worth the writer/publisher's time otherwise.