Take a look below for the equation for spring rate:
I figured I'd post this instead of trying to explain the entire thing.
Also the other thing you can do for cheap is just cut (or have it done as Bill did for me) the stock spring down.
What you end up doing is cutting about a 1-1/4 turns off of the stock coil. I say turns because you're not actually removing 1-1/4 active
coils off of the spring (subtract # of coils that touch from total # of turns for # of active coils)...you are cutting the "dead" coil off plus a bit extra, but by flattening again you gain or consider that a "dead coil".
This effectively increases your spring rate. Because we don't have many coils to begin with, it can be a bit noticeable. I had to dial my compression back a bit with the new cut spring.
But, that being said, it is an effective way, and cheap, to get the bike lowered. Also, if you so desire, using what Kb provided above, you could calculate a spring rate that would work for the bike that would give you what you needed at a lower stance. And see if one of those springs would work, or source one that would.