I notice Flank steak already made the list, and once I would have agreed, but it's gotten trendy, and with the trendiness, expensive. It's still not fillet-price, but it tends to run in the 7-8 dollar a pound range where I live.
The steak I like to grill that I find to be tasty, cheap, and available is skirt steak. Do a light marinade, grill it as lightly as possible, cut across the grain of the meat, serve.
Your mileage may vary, however. The cheapest cuts are almost always the ones that local cooks don't know what to do with, and if the locals are hounds for the fajita, you're not going to be able to find skirt steak at all.
My advice is to go to the store with an open mind, and browse the meat counter looking for deals. I came home with 3 pounds of tenderloin tips for less than twenty bucks the other day: if I'd gone out looking to buy such a thing, I'd have been disappointed.
Best Answer
The main thing you need to do to prepare a plank for grilling is to soak it in water first. Otherwise, it can catch on fire and you'll have a mess.
For seasonings, you can use anything that you would use when roasting something. For salmon, you could put down some lemon slices and rosemary.