Since I don't remember the exact % of bonus XP for the 1 daily low level random, for sake of argument lets say 300%.

I know they did buff base dungeon xp when they did the last round of enhancements but it really does not amount to much or enough.

So using this information let me suggest a few tiers. First of all there should always be a bonus when you use random dungeons by using all dungeons available to character not if you select 2 of 5 and random queue.

X = Base Bonus Rewards
Y = Base Bonus Rewards * 1.5 (+.5 or +1 multiplier if bonus roll type)
Z = Base Bonus Rewards * 3

Solo Queue
Z reward = First time through for each day.
Y reward = Any other time that is completely random.
X reward = If limited selection.

Partial Party Queue (2-3 players pre-grouped for 4), (2-7 players for 8)
Y reward = Any time, individual bonus reward based upon role.
X reward = If limited selection

Full Party Queue
X reward = Any time.