Traditionally Lego bricks cost a lot. This guide will try to give you some guidelines on "brick best price".
First of all basic creative box seems the cheapest one.
For instance the LEGO 10702 - Classic Set Creativo has 583 pieces for less then 25 € on Amazon. The cost per brick is about 4.25 € every 100 bricks.
With this metric, the "Simple Machines Set 6986" at 67€ for 204 pieces is very very expensive, but it is a valuable source for a good set of gears and "hole-bricks" you need to integrate standard lego bricks together: