As the grilling season has (in the minds of retailers - grilling season is year round for us)drawn to and end, check Lowes and Home Depot to see if they might have marked down their Weber replacement grates. If you can find the hinged grates for a reasonable price, you're really in business!
If you're on a tight budget, check Academy for their store brand replacement grates. The wire stock isn't as thick and I doubt you'll consistently get the killer grill marks, but they'll work and around here they're about $5 each. At that price you could use them for a season, toss them and buy new ones for the next year if you needed to.
Pat