Qq plots dont get at linearity. They test normality. Most commonly you use scatterplots to see if the relationship is linear although multivariate linearity is rarely tested (if it even exists) only bivariate linearity. It is common to transform financial data to be more normal with logs (and to address heteroskedacity).
I think you are confusing normality with linearity.
I think predicting future data with existing one outside time series models is pretty dangerous because you violate the assumptions. Most especially it is dangerous if there is a trend to the data, that is the relationship of cost to whatever you are predicting it with is changing over time.
You might want to at least look at exponential smoothing which is pretty straightforward (but assumes past patterns continue).