You can also increase the efficiency of wood by adding blower(s), air injection into the top top of the fire box, inferno arches, preheater, steam-a-way. I think I heard of someone running 2x6 with an inferno style arch and preheater and getting 60 gph. I can't verify this, but I think the most I've seen an oil fired rig do with preheater is 45 gph.

A lot of times you can get free, or near free, slab pine from a local, small time saw mill. I'm getting all my wood that way and you can process it quick. I think I've gotten all my wood processed for 10-20 gallons of gas/oil, and 2-3 weekends of time and the wood's free.

Say you buy grapple loads of hardwood for $1000 per load and figure 25 gal of syrup per chord your looking at a cost of $5 per gallon for fuel, not factoring in your time. Pallets are usually free and tarps are cheap. You don't necessarily need a structure to dry your wood. If you go oil I think it's 3 gallons of oil per gal of syrup so say oil is $2.50/gal your looking at $7.5 per gallon of syrup with no time involved, but when oil was almost $5 per gallon that would've been $15 per gallon of syrup. Wood starts looking more attractive at those prices.