The only guide on here for faceplates was written back in 2006, and is WAY out of date with the current state of the hobby.

First, there is a faceplate database at faceplateaddict.com with almost 1,500 plates from around the world, including customs, autographed, retail plates, preorder gifts, promotional plates and more. Some plates have sold for close to $1,000 while other plates that went for over a hundred can now be had for less than $20. The guide can give you an estimate of the rarity of a plate, although it won't offer an actual price. Ask in their forums if you have a question.

Second, when you search eBay for faceplates, search for "all countries and regions." Many auctions say they won't ship internationally, but the sellers will if asked. Search also "face plate" and "faceplate", look in the stores as well as the lists, and try excluding words like "-skins", "-guitar", and "-case". Be aware that some plates do come with skins and you might kick those out with these exclusions.

Third, watch for shipping, but keep it in context. These plates are 12" long, so with adequate packaging, they can get bumped into the "larger package" catagory, increasing the shipping cost. They're also easily damaged in shipment, so ask for them to be boxed and not put into an envelope, and pay for extra packing material if possible - especially if it is a more expensive collectible plate.

Fourth, make sure you are buying a plate and not a skin. Skins are generally cheaper than plates. I'm not saying they're better or worse, just know what it is that you're buying BEFORE you buy it.

There are very few faceplates remaining in retail stores, so getting it for less at a store is not likely an option. Pay the extra $5 or whatever and get it on eBay, and save a half a tank of gas looking for it.

Ubisoft has made a plate for almost every Tom Clancy game but most of them were only available in either Australia or Europe. EA makes a lot of plates but they usually give them out at tournaments or special events, so many people aren't aware of them. Mad Catz has added skins to their faceplate offerings, allowing them to charge $29 instead of $19.

There are people who will create custom plates for you. Don't balk at the price. You're not buying a plastic accessory. You're buying their artistic skill, their supplies and their time. If they ask for more than you want to spend, tell them that, but don't insult their offer or their abilities.

Here are a few popular collectible plates:
E305 - the first one, with the green rings of light. These are numbered x/5000, but the numbering is on the package. If you're buying it to use, it doesn't matter, but if you're buying it for a collection, get it with the package or wait. $100 with the case, or $50 without.
Bioshock - the plate with the Big Daddy on it was given out to 5,100 Gamestop managers. It came in a box with a lanyard and laminated badge, plus a paper insert. A complete kit can still fetch over $100, while plates run about $60.
Call of Duty 2 - there are many bootlegs of this plate. Bootlegs are more faded and less colorful. The real CoD2, 3 and 4 plates came in a cardboard box (as did the Quake IV plate, which also has a lot of bootlegs). If you're a collector, wait for one in a box before bidding - NOT in a blister pack or loose.
Many plates are ridiculously rare, like the War of the Worlds, Eragon, Moto GP '06, Canadian Launch, Prey and most autographed plates. Major Nelson from Xbox LIVE is very personable and approachable, and has signed a gazillion faceplates. Brad Bird from Pixar, singer Trace Adkins, and Charles Martinet (the voice of Super Mario) have only signed one each. When plates like that go up for sale, don't be surprised if they go for crazy amounts of money.