Bias, along with multi, is one of the two upgrades that make you gain more points per second.
multi_count * bias_count to your powergain.
Bias can be acquired when the point requirement for it is met. Acquiring bias will reset your points to 0.
The cost of bias is calculated with the formula (ladder_number + 1)^n, where n - number of desired bias. For example, the cost of +4 bias on Ladder 3 will be (3 + 1)^4 = 256 points.
These costs are adjusted on [CHEAP] and [EXPSV] ladders - for more information check their respective pages.