A Gravity/Lava Mob grinder with a 25x25 profile from skybox to water level provides me with a decent quantity of TNT, if left overnight in the loot chute. Making it from Cobblestone may prove the bigger issue, as mining out all that crap can take a while, then placement. But I did it over one day (massive Autist mode, did not break much for food or toilet during) and it was creating me drops as soon as I placed the top section on. After placement, you will have to begin clearing out below it (I placed mine above a lake to decrease extra spawning locations outside the spawner) and after one night of sitting on the delivery tray, had plenty of Gunpowder to create several stacks of TNT, sufficient to begin mining operations in earnest.
With mining, you can lose blocks, however with shaped charges, you can choose to open up a 3x3x3, or a 3x3x4 cavity with one TNT charge. When mining the hole for the charge, you get a small glimpse into what you will be blasting, so there is a 50% chance of losing some ore, the speed of blasting however, makes up for this potential loss in my opinion.
For shaped charges, experiment a bit, but I have currently found that mining a placement hole (Not an engineer, or miner, so there is more than likely a correct phrase for it) 4 tiles deep (as far as your pick can reach) and then place one TNT block, and swap to a cobblestone block, activate the TNT, then place one, or two cobblestone blocks in the cavity you made, and dash back to avoid being in the blast radius. Clears out a 3x3x3 or 3x3x4 hole if all blocks are Stone or have Ore. Dirt and Gravel will carry the blast further and can open up a larger cavern.
I used a simple 25x25 tower build that I found here. With my bottom/killing floor just above the water level.
I dont get as much string as I get other mob drops, I suspect the Lava killers and spiders erratic movements to do with this.
http://www.minecraftforum.net/viewtopic ... 35&t=55552
Making it from Dirt seems a bit odd, as the Dirt will carry light levels, I used cobblestone, mind you, I did have Admin on that server, so it was incredibly easy for me to create vast quantities of stone for construction.
Some fast (and ugly) math has it at 889 blocks per spawning floor, 625 for a encased roof, and more per floor. Yes, it may take some time to recreate in Singleplayer with no instaspawn loot from Admin power. But my regular maps have me inundated with cobblestone, now that I have found a productive use for it....
For a game I bought in Alpha, it had given me many hours of joy.