The only way you will save money is if you follow someone else's pattern. If you make your own design, chances are, you will have a first iteration, then a second...etc, until you get it the way you want. The more iterations you do, the less you save.

thru-hiker has a kit, and also free instructions. In fact, the same instructions are on this site as well. There are other instructions online, if you do your research.
Working with down is labor intensive, which is why they are so expensive. By doing the work yourself, you can save considerably, if you plan right.......good luck. I plan to make one myself in the near future.