I figured it was the top & bottom rim armour. Mild steel will definitely cost less, but the reason you don't see it used much is because its not very effective. Is the aluminium plate thick enough to tap a screw in from the side? If you deleted the tabs, and used the spare weight to increase the thickness of the aluminium at the rim, then the ring can be made in one piece and there is just one weld, which will really reduce costs. Having countersunk screw heads on the outside face is a compromise but if the steel ring is thick enough, make the countersink as deep as possible for extra protection. This also allows field replacement without welding, a huge advantage.

Like Garry mentioned, ring rolling is not an exact process like CNC milling. Once those segments have also been distorted by welding on the tabs, you may find that the parts don't fit into the recesses in the aluminium. To fabricate these accurately enough to fit reliably will take a CNCed jig like a section of the aluminium plate. The fabricator may be adding the cost of a jig into the quote.

Another cost is the flush outside bracket; the fabricator will have costed in labour to grind the welds flat. If you can avoid the flush joint, costs will come down.

Is there room to put the bolt in from the inside? avoiding the countersinking in Hardox will definitely reduce costs.

If you have to put the bolts in from the outside, is it possible to use a nut inside? Tapping hardened steel is a real pain and using a nut will cut costs.

That overlapped weld area will make replacement really difficult. If the tabs are recessed and shaped so each segment slides in, then then one end of each segment is trapped under another segment and they will ALL have to be cut off to replace just one. Much simpler and cheaper to just have a but weld between segments.