Commander tournament / alternate casting cost

Players in a commander tournament are trying to claim that when using dash, an "alternate" casting cost is not the same as a normal casting cost, therefore the "commander tax" that would apply to the normal casting cost does not apply to the alternate casting cost. Which is the true case? Is it safe to say this applies to other non-dash abilities?

Dash requires the creature be in your hand. If your Commander is in your hand and it has Dash, then no Commander cost is applied. If your Commander is in the command zone, it cannot be dashed into play.