I just have so much waste oil I could heat my shop all winter. Figured it could be put to good use but some of those heaters are in the 5-7k range. Hard to justify that price.
Check out online forums for ideas to convert fuel oil to waste oil, since most are forced air, you could install ductwork through out your chop. Also one must remember that a waste oil furnace is an investment, with proper maintenance, the unit should work for years. Then sit down and figure out how much money it costs per kw compared to alternatives like propane, natural gas, and electric, you will realize that you will save thousands compared to the others even if need to shell out for the initial cost.
Another example is buying a 500 dollar mattress instead of a 1000+ mattress granted the mattresses will function the same for a period of time but with normal use the 1000 dollar mattress will pay for itself because the 500 dollar mattress keeps you up at night because the support when it was new is worn out.