Anyone else disappointed with the conjuration system in Oblivion? Wouldn't you have liked to spawn multiple creatures? Perhaps some permanent ones? It could be balanced by removing a little of your available mana (so, if you have 500, and a permanent summon requires 75, when you'll summon the creature, it follows you around, but you only have 425 mana to use).
Discuss away!
Into the perma summons, not into the having it give a long term dent to your magicka. The lore doesn't suggest that is required. Do the summons correctly, carefully perform the rituals to bind the pact, huge danger if it goes wrong, but if it goes correctly, viola, no problem.
I would be in support of the Daedra OCCASSIONALLY testing the summoner in an attempt to break free, and at these points there could be a noted strain on the casters magicka (perhaps the bonds of the spell need to be re-strengthened, requiring magicka etc.) but you should not have to walk around gimped as a mage for the entire duration of a permenant summons.