Hi, I'm struggling to reproduce / update these coefficients based on current data - can you please point me to an example / excel spreadsheet with formulas? Or is it an iterative approach?

Thanks.

Put the values into a spreadsheet as Date,Price

You're going to want to normalize your dates per rpietila's convention, where date is number of days since 2009-01-03. In Excel you can accomplish this by taking your column of dates, subtracting the function date(2009,01,03), and showing as numbers.

You're going to want to add a column that is the log base 10 of price. ( =log10() )

Do a least squares on [log base 10 of price] as a function of [number of days since 2009-01-03]. This is a built-in function in the Excel graphing features (there is an option to show you the exact function in y=mx+b form overlaid on the chart itself).

That linear function will now output the log of price.

You can calculate the linear model price by adding a column that's =10^(your least squares function). You could literally create a column where you start "=10^(" and paste in your least squares function, and replace the variable with a cell reference.