Make the 30 damage on hit scale with AP. Maybe 2 damage per 10 ability power. Most supports who build AP cap at ~200 AP by 30 minutes. This will only be an extra 40 damage so 70 damage per hit for 6 seconds at 30 minutes. Just buying ardent censer will already put most supports below the 200 AP range anyways so this buff won't be overpowered (I think).
Ardent Censer is already gold-efficient as is and provides ~1,000g worth of stats to allies, only boosting the efficiency more. It doesn't need any more damage (if it did need a ratio, it'd have to be extremely low, 30 Magic damage adds up quickly for being effortless on many champions. Heck, the attack speed boost alone makes a team suddenly much better at taking turrets. (It's already core on Sona at this point, and she isn't in a bad spot despite others' claims.)
