Cost is a consideration for me and the payback is somewhat secondary but still relevant. You might want to check out the thread on the greenfire furnace that a fellow recently bought for $4400 new. Another option that I am considering is Seton, they offer plans and the owner emailed me that the material cost would be around $2000 plus the plans for $700. Several in the greenfire thread feel that design is simple and could be copied w/o plans. I am not too sure I want to attempt something like that that I don't have too much experience with. Just keep reading through this site there are all kinds of advice and opinions. Just have to decide what works best for your needs, abilities and assests. Guess if I had a million in the bank I would go with a GARN.