Framesets don't really save you money unless you've got a ton of the right parts sitting around at home. For most trikes, that would mean 20" wheels, chains, disc brakes, and lots of drivetrain parts. If you have to buy parts, going that route usually costs you more (although you might end up with a nicer build, too.)

There's going to be a large difference between a Sun and a Terratrike. I wouldn't characterized the Sun as 'Wal-Mart' level, but Suns have only two advantages, IMHO: 1 - they're built to support a 275+ pounder and 2 - any bike shop in the country who's willing can order one for you from their J&B Catalog.