On a one-minute single-boss fight, 1 % spell crit is worth 16.2 healing power and 1 % casting speed 43.1. A point of Intellect weighs 0.35 and a point of MP5 0.01. The weights are measured from the benchmark set by adding a little of each stat, so they price the next few points, not a whole new set.
Per point
- Healing power1.00
- Intellect0.35
- Mana per 5 s0.01
- Spirit0.00
Per 1 %
- Casting speed43.1
- Spell crit16.2
Healing power
Healing power is the yardstick. Each point adds 85.7 % of itself to every Greater Heal and Heal and 19 % to each of Penance's three bolts. Renew gets 20 % a tick, and Prayer of Healing 28.6 % on each member it reaches. On a long fight, though, healing power only matters while you have the mana to cast, which is why MP5 and Intellect sit above it.
Mana: Intellect, MP5 and Spirit
You are casting almost all fight, so you live inside the five-second rule. MP5 counts in full there. Spirit regeneration, 6.25 mana a second plus one for every 8 Spirit, counts only through 
Meditation, at half. That is why a point of MP5 is worth 1.62 and a point of Spirit 0.78 on a three-minute fight.
Intellect gives 15 mana a point and a little crit (about 60 Intellect for 1 %), and Mental Strength and Blessing of Kings raise it by a quarter. On three minutes it is worth 1.11.
On a one-minute fight the mana stats are worth almost nothing: MP5 reads 0.01, because the fight ends before your mana does.
Crit
A critical heal heals 150 %, and 
Divine Aegis adds 15 % of that as a shield, so each crit is worth about 72 % of a heal on top.
For the single-target build 1 % crit is worth 11.5 healing power on three minutes and 16.2 on one. This set sits at 14.9 %.
Casting speed
Pre-raid gear has almost no haste: the 1 % glove enchant, Troll Berserking and the Skyborne racial. On long fights 1 % casting speed is worth 11.8 healing power.
The one-minute weights for it are unreliable: -6.77 for the Troll and 43.1 for the Night Elf on the same fight. A small stat measured on a short fight is mostly the run's noise, so don't trade other stats for it.
On every fight
On two- and three-minute fights a point of MP5 or Intellect is worth more than a point of healing power, and a point of Spirit less. On one minute the weights put the mana stats near zero, and healing power and crit decide.