ah, ok. PCB means Printed Circuit Board usually. I think the acronym you want is BMS (Battery Management System), which itself can be a little nebulous (some just monitor and alert, some override charge, some override discharge, some both, some have temperature monitoring, coulomb counting, etc. etc.)
Usually "budget" means you trade lots of your own time for some cost savings, but you usually get a bit educated in the process (and occasionally see some cost savings, moreso the more educated you get about a subject).
it is just 4s (4 cells in series) though, so if you source 4 4v adapters (electrically isolated) that will charge it balanced (I just have a 16v laptop adapter on my bike, but I monitor the voltage at various points). And a way to stop discharge when the pack is below say 13 volts. But even that isn't completely "idiotproof", but nothing is really.
The most budgety thing you can do is what I did, just hook them up, probe all cells occasionally with cheap multimeter and monitor pack voltage while operating it so you don't over-discharge, and use a $5 16v laptop adapter to charge, and manually correct any out of balance, or take other action as appropriate. But it sounds like you want to sell something with batteries in it, instead of for personal use. I don't recommend doing that without educating yourself more about the hazards (they do occasionally catch on fire, even with a production grade BMS).