I got your point, that you're upset we pay more for our abilities, fact is though all mages pay more for their abilities, it's just a reality of the game(although you could argue that melees pay for theirs by being the ones to get pounded on by the mob).
What fact that mages pay more for all their abilities?

Scholar:
Negative effect of arts is that you are 10% worse at the opposite. Meaning it is a penalty if you attempt to play two roles.
Sublimation drains HP to give MP. Negligable cost since draining 46 HP restores 200 HP with a cure III.

White mage:
You lose HP on martyr to give other person HP. Not exactly a big deal since it is an emergency ability when you have no MP to spend.
You lose HP to give MP on devotion. Not really that negative since again HP is easily restored.

Red mage:
Composure will give you a higher recast. Bad if you are hasting 6 people, but it isn't really those moments you are supposed to use the ability anyway. Pretty much only bad when soloing as RDM/NIN.
Convert takes HP and gives MP. Again, a favorable swap since you can refill HP much easier than MP.

Black mage:
Nothing negative. All JA's have only positive effects. (Unless you want to complain that mana wall drains MP when you get hit)

Blue mage:
Convergence does limit your AOE to single target which could be seen as a penalty. But it sounds more like the whole idea of "focusing the damage to deal more damage to a single target" thing.

Bard:
Troubadour could be claimed to be negative with it taking 1.5 the time to cast songs while doubling the duration.

Summoner:
Mana cede costs 100 MP to give 100 TP. Had it been 100 HP, I'd just shrugged it off. Same deal as for WHM in that case, or for RDM. But MP? Nobody else pays MP for their abilities. NOBODY.
Avatar's favor does a 20% damage reduction to compensate for -2 perpetuation and an Aura effect. In addition each time you use rage pacts, you weaken your aura.

Comparing the above jobs I would claim Summoner got the shortest straw. And I can't understand where you get the impression the other mages are paying terrible prices for their abilities. (And apparently BLM is the winner with not a single negative ability)

I'd gladly swap with most of the jobs. I'd take 100 HP for mana cede cost, that will be a breeze. Then I'll take 1.5 times the recast on spells instead of 20% damage penalty for avatars favor. Now SMN pays as much as WHM or RDM for the abilities, and somehow it is a vast improvement over what SMNs used to pay. How is that possible if all mages pay the same high price?