Making magic items besides the ones listed is a variant option (see sidebar pg. 214 in the DMG). So if your DM allows you to create a ring of Greater Invisibility, you can. As for cost, using the formula for a command word activated item, it would be:
4 (spell lvl) x 7 (caster lvl) x 1800 = 50,400 gp
Note that this is just the result of the formula and should be eyeballed based on the DM's view of the utility. The DMG itself breaks the formula for the Ring of Invisibility, which should come in at 10,800 (2 x 3 x 1800) and prices it at 20,000 gp.
Hope that helps.