Not necessarily. It's typically $15-25. And many used tire dealers (such as this one and this one) offer free shipping, as well as excellent prices.
You can also check craigslist and eBay.
When I switched to a full-size spare, I just used one of my other four tires that was worn and close to replacement for my spare. The thing to remember is, you're only getting a spare tire for use in an emergency, in case you get a flat or blowout on the road. So if it's worn down close to the treadwear indicator bars, that's no big deal; it will still get you to your destination and back, after which you can get your flat tire repaired or replaced.
Along the same lines, here's another idea, which works if you're friendly with the folks in the service department at your local Acura dealer. They probably replace 255/55-18 tires on customer MDXs at least once a week, maybe more. Most of the tires they take off the customer cars probably still have some tread on them (say 3/32" or 4/32" or more). Do you know what the dealers do with those tires? They pay a few bucks for places to dispose of them. I bet they'd be happy to give you one for free, if you asked nicely. It can't hurt to try!