I do some pretty ridiculous things in the name of "saving money, I think." Making your own LME out of an all-grain batch though...other than to say that you've done it, I can't see a good enough reason to attempt this at home.
I had a long drawn-out response all typed up before the big picture became clear. Making LME, while it might save a little money, is essentially a tradeoff of time. Right now, if you have an abundance of time, you can make a lot of wort and boil it down so later you don't have to spend as much time mashing. While producing wort in large quantities will save on prep time significantly versus doing it 1G or 2G at a time (finished product), it may not be realistic since it would require more equpment (2x 10G boilers, 2x burners, a mash tun that can handle ~75lbs grain).
In my revised estimates, I corrected for lower costs of LME and averaged them. In my neck of the woods, LME is only $3.00/lb versus $5.00 in PintoBean's estimates. Assuming a $4.00/lb average for LME at a homebrew supply place, I came up with a potential savings of about $22.00 using an all-grain LME DIY process instead of LME in a 5G batch of 1.090, not including energy/time costs. Energy costs might be around $6.00 per 5G batch of DIY LME, reducing the savings to about $16.00.
Even assuming a potential savings of $16.00, this doesn't include the time investment.
Using the "time is money" argument makes sense in an economic perspective, but in not so much in the homebrew world. I believe few are doing it to truly save money, but instead to make quality beers while enjoying the hobby. In an economic sense, the time spent on brewing seems like it is part or all of the utility that we get out of it since we enjoy the processes.
With that being said, assuming $16.00 savings can be realized per 5G batch of 1.090 beer, and the extra time required to produce that wort prior to brew-day can be found, if no extra equipment is necessary that erases the savings, from a quality perspective, I have to disagree with the DIY LME production idea. When commercial maltsters create extract, they do it with lower heat in vacuum boilers. Extract will naturally be darker since some carmelization occurs during the heating process. In a homebrewer's system, this vacuum will be difficult reproduce, and higher heat will likely be required. This extra heat means more carmelization.
The character of the finished extract will likely be much darker than a commercially purchased extract.
I've never done the DIY extract production thing at home, so obviously I might be full of **** on some of the things I've said. I've only gone so far as to make AG starter wort in bulk and steam canned that. The time required to do that is significant.