It's one of those places that you won't find in an Aires or campsite book as its just a car park with no facilities but a fantastic view. Cost this year was 8 Euros and for that you can park all day and overnight.
If you want more info start with http://www.ot-montsaintmichel.com/accueil_gb.htm but ignore any mention of campsites etc. and just head straight to the mount.
But remember it has no facilities so fill your water tank before you go.