It would cost 625 points.
This would buy you 12 power dice with +1 to cast, which is already a VERY strong magic phase. It would also generate 5 bound spells, which is huge, and 4 breath weapons. The breath weapons might be one use only (and I really like the idea of them) but that's enough to decimate one or two units of enemy infantry. Careful use of those breath weapons will all but guarantee you outnumber enemy infantry in crucial combats, and that can be fatal when much of this army causes fear. As I indicated before, 12 power dice backed up by 5 bound spells is almost overwhelming. How many armies can stop that, or even get close? Add in some of the powerful bound spells floating around the magic item selection and you've got a lot of goodies on offer.
Look at how much plenty of other armies pay for that kind of magic, these characters are not expensive before you throw in bound spells and breath weapons. I don't have my army books with me (except Vampire Counts and they're not exactly typical) but I remember Dark Elves and High Elves pay about 130 for a level 2 (with their respective +1 to cast/dispel) and about 270 for their lord choice.
If that's correct then they'd add up to 660 points, and have considerably less influence on the table (although don't get me wrong... they're still scary!). Sure, the elves have a nicer stat line, but most of the cost of a wizard is caught up in the magical power, and the proposed casters are absolute beasts. I cannot put enough emphasis on how much bound spells change a magic phase. Most armies won't have many (if any) dice left after the bound spells go off... and they'll always go off. If they ignore the bound spells that's 5d6 core troops going on the table, unless the other options are even better at the time (e.g. the free march/charge move). People complain about Vampire Count bunker armies... these guys could definitely do even better (especially given the automatic spell possession).
EDIT: Not sure if a ritual can be used more than once, but considering how the troop generation and movement spells are always going to be excellent, that's hardly an issue. Rerolls to wound is amazing for an army with units of troops who have Killing Blow as well!