It's a win win situation either way if you ask me.
Pre-made kit - someone's taken the guess work out of it. You get all the parts, brackets, hardware etc, bolt it on, tune it, and bam you're done. Usually ends up costing more but it saves you time.
Homemade kit - more legwork on your end rounding up parts, planning mounting locations, tubing work, etc. However...a lot of times when I am installing other people's kits (anything, doesn't have to be f/i) I think, this would be so much better if _______... So you have more leeway in making it as perfect as you desire.
Whether or not you save on cost doing it yourself just boils down to what you feel your own time/labor is worth. When I do things on my own (not just cars but house stuff, whatever) there is also that sense of pride that I did it with my own two hands, that's worth a lot to people too
