When you consider all the various parts of a complete ignition system, as well as the overall design and packaging, the price is not at all unreasonable. CH's single cyl kit system is $89.95. Complete with a weedie timing kit (prop hub & hall effect sensor) is $119.95. Put that into perspective with the cost of other RC related items. A single digital servo can cost that much.

Still, it's always fun to tinker and DIY. Personally, I find it prudent to be frugal elsewhere on the airplane. By shopping around for bargains, closeouts and clearance items, I can easily save more than enough to spend on an ignition system vs. a DIY kit.