I wish I had your problem. My ceiling is already covered and I wish that I had access to the rafters. You can hang all sorts of stuff up in between them to get out of your way. Also makes it super easy to put in hooks for holding fishing rods and stuff like that without need for a stud finder. I guess my point is that dry wall is pretty but rafters are functional.
I, too, wanted to enclose the ceiling in my small (22 x 24) shop, yet retain access to the stuff I had stored on the rafters.
I spent a grand total of $50 for enough 2x4 acoustical tiles to cover the ceiling.
I then spent $10 for a stack of mixed 90°, 3/4" moldings which I am attaching even with the lower edge of the cross pieces.
I need to trim the panels down to 22-1/2" to fit between the 24" o.c. rafters, but I will have complete access to my entire "attic" by simply removing a panel or two.
You can often find a building that is being torn down that has suspended ceilings. Often you can simply have the stuff that would otherwise cost the contractor by filling the dumpster. Other times, you can find the stuff at quite reasonable prices.
In my case, I have a nearby lumber yard/hardware store that buys up surplus, closeouts, and overstocks. My ceiling panels were still in the original boxes, and I paid less than 20 percent of what Home ****-o sells for.