I recommend using Sheldon Brown's gear calculator to find out what you have now, and help you figure out what you might want, and how changes will affect your gearing.

Right now your effective lowest gear (small ring on the front and big in the back) is about 45 gear inches (which is a measurement that takes into account the front gear, the rear gear, and the size of your wheel, and is equivalent to the size of hi-wheeler wheel that would go the same distance per pedal stroke as your bike in that gear). 45 gear inches isn't all that low, if you do a lot of climbing.

A 12-28 cassette would bring your lowest gear down to about 37 gear inches, which is a big change.

Adding a 30t chainring and making your ride a triple would lower your lowest gear to 34 inches, which is barely lower than a cassette change, and would involve the expense of a new shifter, and possibly derailleur.

However, if you can find a 12-30 or 13-30 cassette, you could bring your lowest gear to 34 inches (just like the 30t crankset), but for only the cost of a cassette! Those cassette ranges aren't as common, but they ARE available. Remember that a 13-30 would lower your highest gear a bit too, but it sounds like you aren't as concerned with your highest gears as you are with your lowest.

Anyway. I'd recommend playing with the gear calculator to figure out how changes will affect your gearing. I think you will still find that a differently ranged cassette will give you the most change for the least money. If you need to make major changes to your gearing you could go to a different crankset, but you start the 'cascade effect' which can cause you to have to replace quite a few parts to get everything working again.

good luck!

peace,
sam