Prices are only indicative of the player's growth potential. They absolutely got nothing to do with the player performing good or bad. For those cases, know that 18 yo players might not score so many on their first season. It's alwais good to complement them with older players in their first season (23+). If they don't score, let in your reserve. I had my first striker drop to 3* since when i started playing but kept him playing since he scored and average of 35 goals per season.
EDIT: as for low cost players, if you meant youngsters most of them sometimes comes from very higher level teams dropping out their youth and appearing as 5* to lower teams.