I think you would need a macro somewhere. On loading, it could save your data to a separate page of the notebook, and G8 would contain an @MAX function (assuming that by "maximum value" you mean the highest value yet observed).
Or a simple macro could be written (and stored in a button, or added to a macro that is run when loading prices) that would do what you want:
{If E8>G8}{Let G8,E8}
Charlie