Can't give you the math you need, someone else can I am sure ...
That being said ... and I am a Rams fan from the time they played in the LA Coliseum ... betting on them at this point is a real risk. No Bradford and Clemens is not much in his place at QB ... limited wide receiver capability, no running game to speak of, and the defense most likely will be worn down by The Beast and friends.
YES, it would be nice to middle and collect $1000 ... however, you'll have to put up $600 to win $500 when your teaser is more than marginally alive at -2.5. I've been in your shoes more than once ... more times than not, sitting on what you originally thought was a good bet, is the way to go. (Or you can ask yourself, if STL doesn't cover +14 can I afford to lose $100 ... $500 teaser win minus $600 straight bet on STL)
One last thought ... believe I am 3-1 going into this game on the faux LVH contest here on LVA board ... have SEA -10' and one person is 4-0 with SEA.
Good luck 'Daddy'!