What function should I use?

So I know just enough math to be dangerous, so please try to keep your answers in "layman's terms". I've plotted a bunch of data that represents "productivity" It's in percentage form and signifies what percent that productivity is above (positive) or below (negative) average. I'd like to translate these values into monetary values, but I know it's not a straight line function. If I were to plot it on a graph with the x value would be the productivity value and the y value would be the dollar value. For increases in the lower values of x, y should increase significantly, but at as you plot increases in the values of x, y should increase at a decreasing rate... until some point where for increases in x, y should increase at an increasing rate to infinity (like an S). Negative values of x should operate symmetrically. So what is the base function I should use in order to create this shape graph? Thanks so much! If my explanation isn't clear let me know and I'll revise this.