Hotel parking garages
The hotel garage will often be far costlier than a nearby public garage or other parking option
OK, that'll be £90 for your room, and £40 a day for parking...
Sound familiar? All of a sudden, you're looking at a triple-digit daily charge just for you and your wheels to spend the night.
With rare expectations, hotel parking (whether on-site or using a local garage) costs far more than the municipal garage or public lot. I've actually run into hotels charging $80 a night, just for the car!
The catch, of course, is that if you do pick the public garage/lot, you have to then shuttle yourself between the hotel and said lot—and the most reasonable lots (ones costing under $25 for 24 hours) are usually on the edges of town. Private garages in the historic center will be closer to the hotel, but usually not nearly as cheap (figure on $35 to $60 a night).
Then again, keep in mind that most cities have crazy traffic, arcane driving rules, narrow streets, and a mindbogglingly large percentage of one-way or pedestrian-only roads, and suddenly paying a bit extra to just get rid of the car at your hotel door starts looking pretty tempting. (I once spent nearly an hour just trying—and failing—to drive out of central Oxford while also obeying all the posted road signs; I ended up having to drive illegally down a pedestrian mall for two blocks.)
Contact the tourist office or look in your guidebook to find out the going rate at the city garage, and then ask your hotel what their charge would be, as well as whether it's on-site (garage, lot, or just a few spaces on the streets around the hotel) and DIY, or if it's nearby and requires a valet (and, if so, how much that service costs).
Oh, another random tip: If it is on-street parking (the case with many smaller, cheaper hotels), make sure you tell them you have a car when checking in and exactly where you parked it. The desk clerk will know whether you're in a legal space, and whether he needs to give you a special temporary "residents" parking disc or something so you can run down and stick on your dashboard before getting ticketed.
Back to garages: Long experience—and much driving frustration—has taught me that, unless you're carefully scrimping on every penny, if the hotel's parking only costs up to about $10–$20 more than the municipal lot, it's worth it if only to relieve yourself of the hassle. If the difference is significantly more than that (which it often is), well, then it's up to your take on how much extra time versus money you have to spend, and how wiling you are to get (ahem) taken for a ride on the parking fees.