If we all went by that, none of us would own tanks.....just sayin'.
Well, to be fair, you need to count the number of hours you (and others) have spent on the construction. How many cents/dollars per gallon is it if you value your time at $50/hour?
Ah, well, if we consider the "fun" in the hobby, then we don't need to count the hours spent. However, if there are things that you are doing at aren't "fun," and there are completely manufactured solutions, then you do need to start counting the pennies of your time.
For example, if you wanted to build a 100 gallon tank out of plywood and you considered this "fun" (I would, if I had the time to try it), then you might say that you got it cheaper than the $1/gallon sale tank at PetCo. However, if building the tank isn't "fun" and you are more into raising fish, then that time building the tank takes away from the reset of your life and you need to count the time input in the total cost of the tank.
Just sayin'.
If I were the OP, I think I would count most of the work on the tank as "fun," but this last part has been less "fun," so should be counted towards the cost of the tank. When it became less "fun" and when he wanted to move on to actually raising the fish, only he can say (but there is
a post that punctuates it).