DamonJynx said:
The rules state that it costs the caster 1 MP less to cast the rune on a critical, a 1 MP rune is therefore 'free' to the caster. In the case of an inscribed 1MP rune, the Rune would function as if it had 1 MP invested in it (until the sorcerer decided to de-activate it or he dies, whichever comes first), but that MP is not deducted from the sorcerers total MP. So essentially, you are correct. The item is 'permanently' on.
True, only runes which truly last eternally without reactivation are those carved by Lords of the Higher Planes and those rune items are something a wise person might want to avoid.
However when you inscribe rune to an item with a critical score, it's good to remember that having a rune permanently on isn't necessarily a good thing, because the effect may not be beneficial in every situation and it can also attract some unwanted attention. So it's likely that sorcerer might want to deactivate it at some point and after that activation costs are normal unless another critical is rolled.