... most four-thirds lenses are at least as expensive as the equivalent lenses for full-frame cameras. Since the four-thirds lenses are a lot smaller than their full-frame equivalents, and require a lot less glass, shouldn't they be cheaper to make?
The "equivalent" Four Thirds lens will require less materials if you mean equivalent field of view (half the focal length) and the same minimum f-stops. But most Four Thirds lenses do not match up with 35mm format lenses like that, except the entry level f/3.5-5.6 kit lenses, where the quantity of glass involved is probably to low to dominate the pricing. For example, I have 14-54 f/2.8-3.5 and 50-200 f/2.8-3.5 Four Thirds lenses, but do not see anything like a 28-108 f/2.8-3.5 or 100-400 f/2.8-3.5 lens for 35mm at similar cost.
If you mean equivalent for FOV, DOF and low light handling (allowing for ISO speed difference), the equivalent Four Thirds lens has about half the minimum f-stop too, and so as much glass as its 35mm counterpart. You do not get many options like that either, and when you do, the Four Thirds lens is one of unusually low minimum f-stop, and so a difficult optical design, like the 14-35 and 35-100 f/2 lenses compared to 28-70 and 70-200 f/4 lenses. To me, those super-fast Four Thirds lenses are not very interesting: for high speed and low DOF needs, a larger format is probably the better way to go. "Horses for courses" as Michael likes to say.
If by equivalent you mean same focal length and aperture; no, the format difference has no effect on the amount of glass needed, except with wide angle lenses.