That's a tough one to answer, but you say your "repair guy" gave you the quote. Is he also the AC contractor? Mine would likely say repair it, as a 2 to 4 increase in R value may not help you recover $1,500 in your lifetime. Of course, the overall system will be more efficient so you'd need to factor that in. A good AC guy/gal should be able to run the calcs for you.
What you may want to look at in more detail (if you have a warm room) is the intakes. If they are down the wall they are only as wide as the wall cavity (no matter how large the vent plate is). We improved our cooling efficiency greatly by adding a few around the house through the ceiling. Of course, it was based on the fact that the pressure at the air handler was off (as measured by my AC contractor) and a lot of moisture was building up on the coils, causing corrosion on a 7 year old system.
Also, you may see better than $30-50/month savings from a 25 year old system. I replaced one that old at my rental (1 person living there) and they saw $75 less per month (in central Florida during the summer months).
I guess I'm saying that there are ways to determine what the correct path should be without guessing.