I think a lot of people see the TG as overpriced due to being a special slot. In that category they are competing with Terradons, Cold One Cavalry, Chameleon skinks, Stegadons, and Kroxigors. That is a very congested place with a lot of very effective models that fill roles that can not be filled via other units, we have no other flying creatures, we have no other cavalry, we have no other scouters. the krox, steg and TG end up getting left behind because we have other ways to field that type of unit, we can get elite infantry from the saurus, we can get stegs as character mounts or as a rare choice and krox can be taken in skrox units.
So Temple Guard are not overpriced, just out competed.
All that changes when a slann enters the equation.