Personally if I went to someone and asked them to design me a brochure and I paid them to design it, Id expect to get the files. It seems really unprofessional to hold something hostage just so you can get a few dollars from the print sub out. How can you rationalize charging someone $500 to design but then go and say well its $1500 for the design that you already paid me to do it you want it in any form other than something I printed. If you wanted $1500 to design it, then you should have just charged $1500 to begin with. Maybe they want Tshirts made, or use it in their web design. Are you going to strong arm them into only using you for anything they ever need? That's not a good way to build any sort of goodwill with a customer and also a big risk for them as well. They invest in this design, where what you charged is your choice and irrelevant, then you go out of business or do crap work or any other possibility.
Im not advocating to send every file that you make over to your customer but if you've been tasked with creating an image specific to them like a logo or general layout theme, then Id say its a reasonable expectation on their part to be able to use it without the ransom. Keep them coming back because you do good honest work not out of duress.