While the cost does stay about the same as you level up, once you hit 60 and you start to raise your ilvl, you'll find you get more and more PIE on your gear which increases your maximum MP pool and by extension your natural MP regeneration rate, making using Holy more manageable. Even in lower level situations you should find some MP to use and dropping your MP to even sub 40% isn't a detriment to your role if you play properly (which comes with experience).

Holy should be something you should attempt to use at least 3 times in a single pull just for the maximum Stun duration. Remember to buffer some filler spell between the 1st and 2nd Holy (usually Aero III once you acquire that - Blizzard II makes a decent low level substitute) so the diminishing returns don't negate the 2nd Holy.

Also, something to consider as you become more and more comfortable with Holy - be aware that when partied with DRKs that they have an ability called "Blood Price" which allows them to regenerate MP as they take damage. It's a small purple / pink icon with what I believe is a hand facing down (I think???). This ability obviously runs into issues if the entire pack of mobs is stunned. The rate the DRK will activate this ability will depend on their skill level but is something to be aware of as you get better at using the WHM kit.

Happy "SUPER LIGHT SHOW POWER"ing =p