There is a website that tells you the answer to your seer/cost question.You plug your numbers in to their calculator and compare...I dont know the name of it,but there is such thing.
Next Year 13 seer systems will be lowest available offered for sale.
There are many Houston contrators at this site ,so hang out awhile(my answers aint the best anyway!)
If your satisfied with what you have now,then replace the coil and be done with it,I say.I am sure things are too busy down there right now!
Sometimes there are compounding complexities of multiple variables that are not intuitively obvious