If I were maintaining a list of my party's attacks and their average damages, I would list area effect attacks under multiple headings for the number of potential targets.
Let's take Freezing burst and assume the wizard has a 60% chance to hit and deals 1d6 + 4 damage. Here's how I'd list it in a document.
Freezing Burst (1 target) : 4.5
Freezing Burst (2 targets): 9
Freezing Burst (3 targets): 13.5
Freezing Burst (4 targets): 18
Freezing Burst (5 targets): 22.5
Freezing Burst (6 targets): 27
Freezing Burst (7 targets): 31.5
Freezing Burst (8 targets): 36
Freezing Burst (9 targets): 40.5
That way you can choose which power to use based on how many targets are in the area. This obviously gets a little crowded if you're using a Blast 5
or something like that. If you're using a spreadsheet you can just do something like this:

EDIT: Since your comment indicates that you aren't using this in game, and just for theoretical purposes, you're going to have to make an educated guess as to how many targets are in the area, on average. This obviously depends a lot on many different variables, so your best bet is one of the following:
- Keep track of the maximum number of targets in the area on every turn during every encounter and average your results.
- Ask the wizard what his criteria is for casting the burst. If he only ever casts it if 3 targets are in the area, use that for your calculations.
- Read ahead and analyze every encounter, the terrain, the size of the combat location, the creatures, and their tactics ahead of time and theorize an average number of targets.
If you really want to boil it down to a single number...
...there are two things to look at.
1. The damage potential
This means given any round and any battlemap the average damage multiplied by the average number of targets in the area.
Using #1 above, keep track for a large sample of turns and tally up every time there were 1...9 targets in an optimally placed burst. Use that probability distribution and multiply it with the average damage for the corresponding # of targets, then add your results up.
Example:
1 Target : 77 of 200 turns = 0.385 * 4.5 = 1.7325
2 Targets: 65 of 200 turns = 0.325 * 9 = 2.925
3 Targets: 36 of 200 turns = 0.18 * 13.5 = 2.43
4 Targets: 15 of 200 turns = 0.075 * 18 = 1.35
5 Targets: 5 of 200 turns = 0.025 * 22.5 = 0.5625
6 Targets: 2 of 200 turns = 0.01 * 27 = 0.27
7 Targets: 0 of 200 turns = 0 * 31.5 = 0
8 Targets: 0 of 200 turns = 0 * 36 = 0
9 Targets: 0 of 200 turns = 0 * 40.5 = 0
Sum = 9.27 average damage potential
2. The damage of the spell when used
This one is a lot easier. Keep track over as large a sample as you can of how many targets are in the area only when the wizard casts the spell.
Example:
1 Target : 0 of 200 casts = 0 * 4.5 = 0
2 Targets: 79 of 200 casts = 0.395 * 9 = 3.555
3 Targets: 86 of 200 casts = 0.43 * 13.5 = 5.805
4 Targets: 18 of 200 casts = 0.09 * 18 = 1.62
5 Targets: 11 of 200 casts = 0.055 * 22.5 = 1.2375
6 Targets: 4 of 200 casts = 0.02 * 27 = 0.54
7 Targets: 2 of 200 casts = 0.01 * 31.5 = 0.315
8 Targets: 0 of 200 casts = 0 * 36 = 0
9 Targets: 0 of 200 casts = 0 * 40.5 = 0
Sum = 13.0725 average damage when used