In this kind of scenario its hard to come up with a 'finger in the air' estimate of what the savings could be when embarking on such a project, since the savings are not going to be linear. I was looking for a simple formula that could be generally applied to such projects and came up with this. (a1 is the value to be diminished, and E1 is the 'aggressivity' to apply.
=( SIGN(A2) * $E$1 * (SQRT(2 * (SIGN(A2) * A2 / $E$1) + 1) - 1) )
This can be expressed as a user defined function as
Function diminishingReturn(val As Double, Optional s As Double = 10) As Double
diminishingReturn = Sgn(val) * s * (Sqr(2 * (Sgn(val) * val / s) + 1) - 1)