You'll need to set your payout time. It probably should be the expected length of time you epect to keep the car. The ROI (return on investmen) would be the % returned. If you end up with zero ROI or any positive number, then you are ahead of the costs.
CAI isn't probably going to help MPG. The K&N you already have ordered will help a little. You really need to test that as your only mod, till you have the data on that.
Exhaust might help MPG a little, but it's an expensive item.
I think the extended change intervals for Amsoil are over hyped. Using a Castrol or Mobil synthetic should do just as well, since your car already has some miles on it. Most of the premium oils are "gas saving", but the difference may be too small to measure.
As for the dual oil filter, that's not going to help MPG, and with such an installation you are iinviting the chances for leaks and other problems. I don't know what it costs, but the payout may not be there, even if you do go with the extended oil changes.
The only thing that I can see that would really boost MPG would be a change in the differential gearing to a higher (lower numerical) gearing, allowing your car to run at lower RPM on the highway and in top gear. I don't know if such is available, but that would be the biggest change you could make. IMHO