I priced it up a while back. Once you have purchased a decent battery spot welder and the battery management boards it works out around the same price as buying one plus your labour. Half a dozen starts to look good, but then if your selling them you need to offer support. Not looking so good again. What I did realise was that for the amperage offered by a commercial battery you could get that in a lot less space. Then the penny dropped as to how they seem to perform so well. The electronics never allow them to discharge to their true capacity. So you have the appearance of stable voltage and many charge discharge cycles. Also you don't need to use grade "A" cells. So really a commercial lithium battery should be much cheaper.
I plan to build a lithium battery at some time in the nearish future, but only really for an 'interest level' reason.
But for the various reasons from the Squirrel Cooker, I don't regard it as a money-making or money-saving project.
You're going at it the wrong way. 18650 cells are not the correct chemistry for leisure batteries. What you want to look at is Lithium Ion Phosphate, or LiFePo4, and you can buy these in a large size called prismatic cells. Start here: