This really depends on a lot of different variables. It's pretty unlikely that an upgrade before the useful life of your air conditioner is over will be cost effective. You could calculate this out but it'd be quite the process. You'd start with something like this (
straight off of Wikipedia):
The annual total cooling output would be:
5000 BTU/h × 8 h/day × 125 days/year = 5,000,000 BTU/year
With a SEER of 10, the annual electrical energy usage would be about:
5,000,000 BTU/year / 10 BTU/W·h = 500,000 W·h/year
The average power usage may also be calculated more simply by:
Average power = (BTU/h) / (SEER) = 5000 / 10 = 500 W
If your electricity cost is 20¢/kW·h, then your cost per operating hour is:
0.5 kW * 20¢/kW·h = 10¢/h
So here you would use whatever your electricity cost and usage expectations are to figure out the cost over the next 8 years (15 year expected life - 7 years of life used). Also keep in mind that electricity prices will continue to rise so that's a variable that you'll never know for sure.
The next part would be figuring out how much less the 14 or 15 SEER unit would cost you by computing the same thing again using the new numbers and adding the upgrade cost into that equation.
Another consideration is how long you plan on staying in the house. If you plan to move before the current unit needs replacement and the cost of said replacement won't be on you then there is even less of a reason to upgrade.
To be honest, there are very few situations in which replacing an AC unit is a worthwhile investment unless facing a major repair or the end of it's expected lifetime. Like InspectorD said it's a much better idea to focus on areas that you are directly losing cooling and cost much less to fix.
