With the globalisation of manufacture, I guess there's a handful of places where Lipo cells are made. QC decides then what goes where. Brand name batteries probably pay top dollar to secure the cells they want and make sure they are within the parameters, the rest goes to lesser companies and so on all the way to the rejects which end up in cheap packs. This being my picture I believe you can be lucky and get a good cheap pack but it's not going to be phenomenal, and you can't be sure the next identical one you buy is going to be as good. The price then reflects the parameters imposed by the company making the packs, not the one making the cells.
Sure, on the other hand technology is so good that it is likely the top product will not be much different from the bottom one (bar perhaps backyard made crap) as is the case in many other industries.