$3800 is probably fairly close to other mfgs. I would assume that price includes insulation though. I would check on that.
For brand new materials on just the furnace, propane, and accessories(no insulation), you're probably around $1K.
New furnaces are around $600, Like @srt20 said, you can get used ones for $300 on CL. Can usually always find someone parting out a camper.
I had about $600 into mine. No insulation. It works great for quick repairs.
But if you using for longer periods, or drying out riding gear/sleds, you definitely need it insulated. My next trailer project begins in two weeks. Fully insulated including floor.