That jar markup is mostly glass, shipping, and someone arranging spices on a shelf. I keep a list of what I buy where, and it's changed the most for whole spices.
For whole cardamom, Patel Brothers in Jackson Heights is hard to beat. Yes, the bags look big, but you don't have to buy the jumbo one—there are smaller bags near the front, and you can decant into old jars when you get home. For smaller quantities of specialty things like amchur or fenugreek leaves, Kalustyan's is more expensive but useful because you can get just a few spoonfuls. Sahadi's on Atlantic is fine, but I wouldn't call it cheap for basics unless you stick to their pre-packed bags.
My real advice: buy whole spices, store them in the freezer, and grind small amounts as needed. Whole cumin, cardamom pods, black peppercorns, cinnamon sticks—these hold their flavor for months longer than pre-ground. I keep a sharpie in the kitchen and write the date on the bag. It's saved me from using stale coriander more than once.
If you're already in Flatbush, check the Caribbean markets for allspice, thyme, and bay leaves. For cardamom specifically, a Jackson Heights run is worth it if you combine it with a produce stop. Just go early on Saturday or on a weekday afternoon, because the 7 gets rough after 11.
One caveat: at any open-bin spot, smell before you scoop. High turnover beats a famous name. A busy South Asian grocer with dusty-looking aisles is often fresher than a fancy shop that hasn't sold cardamom in a month.