I think the last BCG I bought was a Toolcraft nitrided for $70 on sale from Right to Bear last fall.
AIM Surplus also has good bcgs in just about any coating or configuration you can imagine. Lightweight nitrided BCG? Yup.
Free shipping too.
AIM also has bolt back up kits. Complete bolt, firing pin, cam & retaining pin. Not that much cheaper than a budget BCG but small enough to fit in a grip with storage. Having a complete BCG is a fast swap but I’ve never seen a carrier fail. Bolt, yes. Pin, yes.
The best bolt IMHO is the LMT enhanced bolt, I have one, nice piece and has a pretty good reputation, but nowadays I can buy more than a few good bolts for what one LMT runs these days. Colt, BCM & Rock River Arms make better than average bolts as well.
The Ruger BCG is good.