Trendline become a function?

Is there a way that I can turn a chart's log trendline into a function, so that when the scatterplot data changes, and the logarithmic trendline equation changes, the function automatically changes and produces new results? I've tried creating a macro where the equation from the trendline is copied and pasted into a cell and linked to data (demand levels) to create a column of results (corresponding prices), but the macro pastes the old trendline equation even after the chart updates and displays a new trendline equation.

Thanks for the suggestion - unfortunately, I have only the beginning and the end data point, so when I choose LINEST, it offers me a straight line connecting them rather than the log curve I need - and the LOGEST function doesn't work because it doesn't allow me to select the specific x's with which to offer me a log based y. Not sure if this makes sense, but here's my example, to maybe make it more clear. I know that when demand is very low (1000 units) I have to charge a relatively high price to cover my costs (1.10/unit), and I know that when demand is high (50,000 units) I can charge a much lower price (0.50/unit.) What I am trying to do it create a set of data that offers me all of the prices along the natural log curve between those two data points - and that automatically updates when my low demand changes to 2000 units or my low price changes to 1.20, due to a different (more or less expensive) product offering or a different customer's demand profile. Does that make sense? The graph with the trendline shows me exactly how each price/demand curve should look, but I can't find a way to actually show the number values related to the other points along that trendline (not the beginning or end), either on the graph or outside of the graph.