Steamers

If a horse favourite shortens in price from 4 to 1 into 2 to 1 if you have layed it at an average of 3 to 1 is a break even scenario over the long run and if so if you can lay it at an average of less than 3 to 1 is that a profitable situation over the long run ?

Or are they both losing propositions just playing with a laying favourites system trying to get a feel for it get some idea