I design and make most of my own sails, it's a heck of a lot of work and engineering, and very easy to get it wrong. I only do it because it's part of my hobby and I like using my hands and mind designing and making stuff (I'm a design engineer and master craftsman). I've designed and built many things just for the fun of it ( helicopters, racing hydroplanes, an airplane, many race cars, etc) all just for the fun of it.
If I counted my engineering time and labor hours, my own sails cost me triple what a commercial sail costs. Thats not including the $40k computer system I design on and the very expensive sewing equipment needed to sew sails (my Pfaff machine was around $2k), plus the huge layout area and tables required.
If I had to guess I would estimate 75% of a sails cost is the cost of the sailcloth and raw materials, unless you plant your own dacron trees in your yard and grow your own( LOL) and get a loom, you can't work around that cost. I read every book I could find on sail design for a year or two before attempting to make my first sail.
However if this interests you as a hobby there are several places to start.
A company called Sailrite has very good instructional videos and sell pre-cut panels and designs that you can make yourself. ( http://www.sailritesails.com/videoselections.aspx
) They also sell all the materials.
There are free sail design programs available on the web (just google sailcut CAD) or go to this link ( http://www.sailcut.com/Sailcut_CAD
). Most local colleges offer engineering programs in fluid dynamics if you really want to get into it, many colleges offer degree programs on the subject. You could offer to intern at a sail shop, but I would think you would need a formal engineering education prior just so you can do and understand the math and concepts (tons of math involved). Otherwise you will end up being a gopher and delivery driver for them. Hope this helps you.