Great advice all around. Cardamom is a common spice in Indian cooking and should be less expensive at Indian markets. But keep in mind that cardamom is the third most expensive flavoring in the world behind only saffron and vanilla. If you don't mind the work, whole cardamom pods will be less expensive than pre-ground cardamom. Scrape the seeds out of the pods (a laborious task--that's partly why pre-ground cardomom is so expensive) then grind the seeds yourself in a spice grinder, clean coffee mill, or with a mortar and pestle. The bonus: a fresher, more intense aroma. Also, the whole pods make a great flavor addition to simmering liquids for cooking rice, poaching fish or chicken, etc.
Nothing tastes quite like cardamom. It's a relative of ginger, but has a musky aroma alluded to by panini (in this thread). For a somewhat similar effect, you could use a mixture of cinnamon and nutmeg (or cloves or allspice) as a replacement for ground cardamom in a 1:1 ratio. The substitute won't taste exactly like cardamom (the musky aroma will be missing), but you will be using similar "warm" spices in place of the cardamom and the recipe will not fail.