The variance of the OLS estimator is VAR( ^B)=σ^{2} /ns^{2}_{x} , where s^{2}_{x}=£x^{2}/x You're hired to estimate and you're going to be paid according to the accuracy of your estimate. Specifically, your revenue is ¥/std (B). You can buy as many observations as you want at price . The sample variance of , never changes. How many observations should you buy?