The DIY cost is pretty reasonable. But the cost would be even better if there was an alternative to the expensive lithium-ion, which in my opinion is dangerous due to the strong emf field they give off and their propensity for exploding unexpectedly. Of course I suppose that last down-side, compared to the volume of them in circulation, is low. Still, I don't carry a cell phone around for those reasons and wouldn't like to be sitting on top of one for even short periods if I can avoid it.
Guess there's nothing that doesn't have a downside to it.