What I would do if it were me would be to make 20 gallons of all-grain wort using a base malt such as marris otter or golden promise at a gravity of 1.090. Then, split into 2 seperate kettles (10 gallons each), boil each kettle down to 2.5 gallons using a gentle boil (as not to scorch or carmelize as much) so you're left with 5 gallons total at a gravity of 1.360. Pressure can into 20 quart size mason jars, and save for later use. It will be a days worth of work to boil down that much wort, and I'd have to run some more calculations on the cost of doing that vs the cost of buying LME, but it'd be a fun experiment. Heck, I might do that now that I've thought of it.
After a quick calc, this would take ~73.5lbs of grain, at my prices for Golden Promise, this would be $92 worth of grain. That would break down to $4.60/qt of the high gravity wort. Wort at this gravity would weigh 2.8 lbs per quart. If added to 3 more quarts of water to make a gallon, you'd have a gallon of wort at a gravity of 1.090 ( [0.25*360 GU's] + [0.75*0 GU's] = 90 GU's in 1 gallon). So, the extract potential for one lb of the wort is 90/2.8 or 1.032 in 1 gallon...not sure where I'm going with this...but, for ease of understanding this, lets take making an equivalent beer from the high gravity wort and LME..and DME for fun.
For ease of calulation, lets say 5 gallons of 1.090 wort.
*High Gravity Wort
(14lbs wort / 2.8lbs/qt) * $4.60/qt = $23.00
*LME
You need just shy of 12lbs of LME to make 5 gallons of 1.090 wort
at a cost of $14.95 for 3.3lbs from my LHBS, that's ($14.95/3.3 lbs) * 12lbs = $54.36.
*DME
In this example, you need a scosche over 10 lbs to make 5 gallons of 1.090 wort
at a cost of $12.95 for 3 lbs from my LHBS, that's ($12.95/3 lbs) * 10 lbs = $43.17.
Someone check that, my brain is fried, even for an engineer.