Surely a portion of the added cost is due to the lower volume production runs. They probably sell the 2.5's 5:1 over the 3's. That means raw material volume discounts will be considerably better for the 2.5's. Material removal is also considerably less with the 2.5's. Consider the added machining time required to make a 3" shock's piston vs the 2.5", or turn the thread on the body. Huge difference there. Machines are billed hourly, so they cost production runs in-house just as an outside manufacturer does.
I work in micro-machining, but the same principles apply. You're close by. Ever see Sorenson Engineering on the north side of the 10 just past the crest of the summit in Yucaipa? American flag on the roof of a polygon/round building?