I have experienced much the same as you, and I'm not surprised in the slightest. At first there is interest in your photo, but when the subject of a fee is brought up they beg off. Nevertheless, they have come to you because you have something they want, and they will probably steal it if you don't give it to them for free.
I suspect that soon I will be engaging a lawyer to pursue those who pirate my images, at least in the print media. After all, a website can remove your image and apologize, a book publisher can't do a thing once the product leaves the presses, so they are more vulnerable to your legal efforts.
Many inquiries come from people who claim to be working for a corporation on a project. If this is true, then there is a budget for this project. No projects are done for free in the corporate world, and none are run from the petty cash drawer. Even non-profit organizations have budgets, and payrolls as well. Don't give them your work for free.
Do the research, develop a professional looking price schedule you feel is fair,(fair to YOU!), and send it to them with your reply, after thanking them nicely for their interest. Do not hesitate to charge for an image no matter if you deliver it on a floppy or CD in the mail,or if you send it my email. Charge them for shipment.
I care when someone steals my work. How do I know when this happens? Simply by going to a search engine and typing in my name reveals some of the thievery. Every 60 days or so I find unauthorized use of my photos on websites in this manner. Approach these theives with your price schedule, thank them for their interest in your work, and highlight the fee that applies to them. You won't get paid, but your photos are likely to dissappear from their website in a day or two.
You aren't likely to get the cost of film covered by sales such as this, so you'll need a day job. But hey, good shooting to you.