Actually the mana costs are relatively the same since Dispell is 14% of priest base mana and Arcane Shot is 135 mana for the lowest rank that dispells (rank 6) and 230 for the highest (rank 9). Purge may be cheaper at only 8% of base mana and Mass Dispell is 33% of base mana which is no where near 33% of total mana. It does do damage which is nice, but you can't spam cast it because of the 6 second cooldown (5 with talents). It also only removes one affect whereas Purge and Dispell both remove two and are spamable. Mass dispell also removes only one, but it can hit 5 friendlies and 5 enemies at once for a total of 10 spells (5 buffs and 5 debuffs).

In your hunter vs priest case you could argue more in favor for the priest if you included talents like you did for the hunter. Example: Pain Suppression will reduce all damage by 40% and makes buffs 65% more resistant to dispels on a 2 minute timer. You also have Power Infusion which increases spell damage and healing by 20% on a 3 minute timer. Imp Divine Spirit increases spell damage and healing by 10% of spirit, Silent Resolve makes all spells 20% resistant to arcane shot's purge affect, and stacking the shield talents gives you a 15% stronger shield and 50% damage reflection. Don't forget 15% cheaper mass dispel that becaomes a 0.5 second cast with Focused Power and Absolution. Personally I like Martydom since it gives you a 6 second immunity to interruption when you're a victim of a critical strike and also increases knockback resistance by 20%. If you're going against a BM hunter the pet will probably crit to get flurry and if you stack Martyrdom you'll be immune from pet knockback. If you're a discipline priest you can also get Focused Will to stack with the pet. Permanent -15% damage and +30% healing recieved is a nice way to stay alive.

Will that help you kill the hunter? Well...no. You're playing a priest for goodness sake. They are effective counters though and if you've got a group. Of course if you're playing a shadow priest then its meltface -> collect honor.

I like this build though if you want to melt faces against a BM hunter. http://www.wowhead.com/?talent=bxxMzZZVMgtMdRhtEo

Shadow to melt faces and Martydom to give you focused casting on crits with silent resolve making your buffs 20% undispelable. Though I suppose you could just throw up Inner Fire to eat the arcane shot purge and spread talents elsewhere. Like Imp Fort and Imp shield or something. Blackout gives you some CC since its a 3 second stun whenever any shadow spell hits. I know shadow priests like to spam cast SW:P rank 1 for 25 mana for both shadow weaving, misery, and blackout procs. Its instant cast too so you can spam it to get the 10% proc rate.

If you want to do support priest you can use http://www.wowhead.com/?talent=bxhMuhgtrcqtfZV0b. Improved mass dispel speed and cost. Blackout and Shadow Focus for the SW:P1 spam stun. Improved healing focus with martyrdom for reduced knockback with healing. If BW is used then turn on Pain Suppression to negate the bonus damage which can then stack with Focused Will for 80% damage reduction (though if you're ignoring the pet that's 15% damage reduction already). Apply the shield for reflective damage and a possible sacrifice for purge. The point of the support role, however, is for arena combat to negate paladin bubbles with its 0.5 cast speed mass dispel and improved survial. You'll probably be aimed shot since you aren't actively trying to kill the hunter and they will have the 3 seconds to wind up (unless they're being attacked at which point they'll suffer knockback as if they were casting a spell). Another alternative would be to mana burn people in order to eat away at mana pools so they can't use those abilities. I remember us having a bit of success using a hidden priest just mana burning when heals weren't needed yet or if we had two healers so that you're doubling the mana cost of the other side's heals by taking away healer mana and forcing them to heal themselves.

Ah so many options and counters when you consider talents. Flared was right, priests must be overpowered.