Most of the resources needed to make obsidian are resources you either have in such abundance you can't even use them up (cobblestone) or are already required items for early-game quests that you should be completing before you even think about making any tanks.
So, let's just itemize the damages.
7 clay, 7 bone meal: Crucible is *absolutely* a required component of early game. This is a non-issue due to absolute necessity of the product as well as re-useability.
5 logs: Er... what? Where? 2 sticks to make a stone hammer (already a required part of early-game quest progression) which has over a hundred uses (you need only 12), which makes almost the entire resource cost negligible due to re-useability. Two more sticks to smelt the crucible. I'm failing to see where 5 logs are coming into play here.
3 iron ingots: An early-game quest has an optional reward of 8 iron dust. An iron bucket is essential to early-game, therefore the re-useability of this resource makes the cost a non-issue.
Cobblestone: Craft a Extra Utilities item transfer node, cobblestone and lava are now free and *very* rapidly replenished. Because a cobblegen is a complete no-brainer for non-challenge playthroughs, this makes any cobblestone cost negligible as soon as a natural cobblegen is created (in the first 2 minutes of a new world, if so desired).
You may notice I just made the entire resource cost except for a small fraction of two sticks negligible. Just throwing a bunch of item frames full of cobblestone up on a wall doesn't show even half the relevant data. If you want to make tanks before you make a cobblegen, or go really crazy and play a no-cobblegen challenge, then there's some resource investment involved. Otherwise? A few minutes of your time, and need I point out that an obsidian generator is a *really* good thing to have for other reasons? Thus even the time investment is a non-issue because of the value of obsidian plates.
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DoomSquirter No, Special Mobs really *is* badly coded. I'm fully justified in griping about a badly coded mod. Its features are not to my tastes, but do you hear me complaining about creepers that heal their friends, or ghasts that drop brutish zombies inside my compound? No, you hear me griping about bad coding practices. If you want to hear more griping, ask me about my trip through HarvestCraft's decompiled source sometime. At least crops are low-maintenance enough not to choke my CPU with constant pings, like an army of sonar operators on crystal meth.