Efficiency factor is missing, and your price of gas is off.
Gas is usually ranked from 85% to 97% efficient -- how much you pay for converted to usable heat in your home.
Wood is usually ranked from 60% to 75% efficient.
Worse case would be going from a modern high efficiency (97%) forced hot air natural gas system to a mediocre EPA stove (65%).
25 thousand c.f. x 1M BTUs x .97 == 24.25M BTUs
24.25M BTUs / (21M BTUs/cord x .65) == 1.7 Cords
$250 for a cord of red oak, seasoned sounds reasonable.
$95/mcf (1000 c.f. ... like timber, they use "m" for thousand...) is WAY too high though.
Natural Gas Prices - Energy Explained, Your Guide To Understanding Energy
$20/mcf is probably a more realistic price. (And usually retail natural gas is sold in "therms" which are the volume that contains 100,000 BTUs, since natural gas varies a bit...but 1mcf is always close to 10 therms)
So for equivelant heat delivered:
25mcf x $20/mcf == $500
1.7 cords x $250/cord == $425
Editing to add oil comparison:
No. 2 Home Heating Oil: 140,000 BTUs/gallon; 85% efficient (if you have a good furnace!)
24.25M BTUs / 140,000 BTUs/gallon == 173 Gallons
173 Gallons / .85 == 203 Gallons by the time you account for inefficiency.
203 gallons * $3.50/gallon == $710
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In my own case, I already own the pickup and land whether I burned wood or not.
I figure my current cost for heating with wood is $35/cord -- that's accounting for my stove, the chainsaws I used, Fiskars ax, Logrite Peavy (all spread out over 10 years), and fuel/oil and chain costs. In reality it's a bit higher from stuff I've bought over the years no longer actively used.
When I put in a more efficient stove next year, I'll cut my wood consumption down plus have to add in the cost of the stove over say a 15 year estimated life...it'll probably raise my cost per cord to $50-60...but I'll cut the number of cords in half. Of course if I can turn around and sell those two "saved" cords each year that would offset my expenses...