My current set of armour is +2 glamour mithral half-plate. I would like to add Restful to the gear but I cannot figure out if I pay the Price +4,500 gp or if it costs more to add on as I cannot see what kind of + it is to the gear.
The price to add a non-plus magic armor, shield, or weapon special ability is typically the listed price of that magic armor, shield, or weapon special ability. Thus the price will usually be 4,500 gp to modify your +2 mithral half-plate so that it's +2 restful mithral half-plate. (This assumes no extra costs are mandated by the GM or incurred during the campaign, of course.)
So far as I'm aware, the game never specifically says that this is how flat-costed magic armor, shield, and weapon special abilities should be priced, but, since flat-price magic armor, shield, and weapon special abilities aren't given pluses and have that flat price listed, using the listed prices has always seemed to this reader—and everyone he's played this game with—how these things are done.