If you add a recall ability and the new summon changes (summons try escape with you) it may be possible to keep it alive for some time. Making it "permanent" is mostly a matter of flavour though; as it would be nice to (at least in theory) be able to keep a powerful clone around.
The steepness of the price would obviously be a matter of balance (make it 10%, only reduce HP/MP, etc.). But the basic idea is to make the cost permanent, making it more of a panic-button (or possibly a permanent friend) rather than a spamable ability.
I also support the idea of making it a high-tier god ability, in which case a big piety cost could be added. It would also make it easier to balance, as you can balance the god as a whole (rather than having to balance it as a single spell against all other spells)
@Psieye, if you want to get technical: Mara's copy isn't
permanent