If I add an SSD to a non-retina MBP 13" will weight drop to 3.5 lbs
If you upgrade the 13" non-retina MBP to a 256 GB SSD, that only saves you $100 vs. a retina MBP with the same 256 GB SSD ($1599 vs. $1699).

Furthermore, the non-retina MBP will still have the optical drive no matter what (you can't configure it without one). So the only difference in weight will be the difference between the SSD and traditional HDD, whatever that is (I suspect not much, since Apple's own tech specs only say "actual weight varies by configuration").

The 13" MacBook Air is a much better fit for what you're looking for: lower price ($1299), much lower weight (2.96 lbs).