This mod makes AWESOME Sink coupons cost the same amount of sink points. The cost can be set per track (currently, only Default and Exploration tracks exist) in the mod's config settings. No, this won't make coupons free (this would result in an infinite loop, which is bad). Multiplayer support is currently untested, but Should:tm: work.

But if the coupon cost is constant, why is this mod called "Linear Coupon Cost"? Well, the accumulated cost of each coupon with this mod does scale linearly. This is easy to see when one changes the coupon cost after having sunk something. Raising the cost results in the "points until next coupon" value dramatically skyrocketing, whereas lowering the cost can make the "points until next coupon" value become negative (until something gets sunk, which makes the value update and grants any pending coupons).