|Divine Light||11733 + 1.15306p|
|Beacon - Divine Light||5866 + 0.57653p|
|Light of Dawn (Glyphed, 3pt)||11512 + 2.376p|
|Beacon - Light of Dawn||5756 + 1.188p|
(Values taken from Elitist Jerks.)
For the on-beacon scenario, healing done = 3 DL + 1 LoD + 1 Beacon_LoD.
For the off-beacon scenario, the extra GCD contributes about 60% of a Divine Light. Healing done = 3 DL + 3 Beacon_DL + 60% * DL + 60% * Beacon_DL
If we remove the common 3 Divine Lights from both scenarios, we end up with the following:
On-beacon healing: 17268 + 3.564p
Off-beacon healing: 28159 + 2.767p
Note that on-beacon healing actually has a higher coefficient. What that means is that there is a point of spellpower such that on-beacon healing will surpass off-beacon healing. That point is around 13700 spell power.
So off-beacon healing still does more healing. But there is a cost. That 60% of a Divine Light costs a fair bit of mana, while the Light of Dawn was free. And on-beacon healing will do more if the last heal in the off-beacon scenario was a Holy Light instead of a Divine Light.
So, unless my math is greatly wrong, on-beacon healing is surprising close to off-beacon healing. It may even supersede it if the off-beacon healer casts Holy Lights sometime during the fight, as the mana saved during the on-beacon rotation can allow you to replace some of the Holy Lights with a Divine Light.
Of course, this doesn't count the practicalities of healing specific fights. Sometimes two tanks are taking the brunt of the damage, weighting towards off-beacon healing. Sometimes only one tank is taking damage, weighting towards on-beacon healing. In a 10-man, Light of Dawn is less likely to hit all 6 targets (though it does hit pets as well).