For my twopenneth, I always go for highest star rating, but, I have bought some duds by doing this. At the level I one of my teams is at (l31) virtaully all players cost 100m. I have personally put players on the market at the lowest available price just to sell them, so the price does not always reflect the quality of the player but may reflect how desperate the manager is to sell. I also always check his stats if they are available.