Without knowing the exact Kepler models, I would think that the 550Ti is right on the cusp of being rebranded once Kepler comes out. Aka, the same card for the same price but named 650Ti. Take for example the AMD 5770 to 6770 rebrand off the top of my head. The 5770 was a very popular card in this price range and was rebranded as the 6770 when the 6 series AMD cards came out. I know Nvidia does this also, but I don't have any specific examples.
Granted with the new 28nm fab process, it seems that rebranding at the old 40nm process is only going to be happening in the low end cards this time around. If your card doesnt get rebranded I would expect its "comparable price" to drop into the $80 range when the new cards come out and the new-release prices settle down. As a general rule of thumb, I would say ~$200-$230 cards tend to hold their value the best through multiple generations. Also, your 550Ti isn't a speed demon even by current standards.
i5-3570K, ASRock Z77 Pro4-m, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB G-Skill @1.25V, Silverstone PS07B