These are just my best guess based on recollection. I also built this system a few years ago, so prices might have changed.
-For a 2p4s (8 cell) 12 volt system-
-My BMS was about $100
-10 gauge silicone copper wire was about $50 for the amount I needed (I used multiple conductors run in parallel for some portions of the battery). That being said, the company I ordered from screwed up the order and sent me some extra wire for free, so it could have been a little bit more than that.
-A crimping tool, good adhesive heat shrink, and connectors were about $50.
-I used two pieces of flat copper bar to make bus bars, they were about $20 total.
-The battery housing was made with scrap wood and a few bolts, maybe $15 in total if I were to pay for it. (This does not include the case that holds everything, just the pieces that hold the battery cells together)
-40 amp DC-DC charger was about $150 (allows me to charge from just about any source)
-I use used server rack power supply's as chargers, they were about $30 for two 70 amp units at 12-15v, but required some easy to do modifications.
-A good quality MPPT controller was about $125, I got it open box.
-I had an active balancer, it was about $20, but it's no longer on the system because my cells are pretty well-matched.
-Class T fuses we're all purchased used, for between $5-$30. There's probably about $80 of fuses in there.
-Small automotive power distribution box was about $15.
-Shunt was under $20.
My guesstimate is about $675 for a 12v system, not including the batteries themselves or the case and inverter. You can add an extra few hundred dollars for parts that I tried to cheap out on, and now live in a box, and ended up just buying a better quality component.
Sorry the numbers aren't exact, but it's been quite a while since I put it together, and I purchased a lot of items on sale or used.
All of that being said, every part is user replaceable, and can be upgraded or downgraded at will.