It can be fairly cost effective (and that's a relative term in aviation) these days with the new sleek designs that offer light weight and min drag. Aircraft diesels are poised to make a big scene in the GA arena with gas prices where they are. It's very possible to get 150-275mph cruising speeds for under 12gal/hr (with full glass IFR glass cockpits). Run some numbers on that if you travel X/C alot and see how it compares to the cost and frustration of an Airline. It's exciting to think about the possibilities out there. Can't wait to build my own someday as well.