Value of artifacts.

Nah, it's fine. Look at the stats of other artifacts — majors give +5 or +6 to parameters (the dragon torso in ER is a relic).

And relics start from +8 (take the pieces of the titan set, for example). So the sandals fit right into this pattern.

The trouble with the sandals isn't even that they give little, but that the bonus is spread across parameters, so there isn't such a clear effect as, say, from the cuirass.