In one of the 3.x.x patches they updated riding trainers so that they'd give you discounts based on your reputation with them. The gotcha is that the most readily available trainers - the one sitting on Dal and the ones in SMV you got used to going to in BC - aren't members of any faction and, therefore, don't give a discount. In other words, if you're lazy you get screwed. Why?
Well, if you fly out to Hellfire Peninsula or Borean Tundra you'll find Flight Trainers near your Flight Masters in the starting fortifications of these zones. These FT's are members of specific factions (Honour Hold/Valiance Expedition for the Alliance, Thrallmar/Warsong Offensive for the Horde), and, given that you should at least be able to make Revered with the former of those by 70 and Exalted with the later is easy to get grinding dungeons sans tabard at 80, scoring a 15-20% discount on your 5K Artisan (280%) and 5K Master (310%) Riding costs is a breeze. So, for a little Flight Time, you're saving 750-1K Gold on each of those; a nice little sum of money to keep in your pocket for Cataclysm and really adds up when you've multiple toons to buy Master for.
You might also want to remember this when 4.0.3 hits and you've gotta put out 250G a toon for "Old Weather Flying"
PS: For those that don't read patch notes: all your mounts that ride now go the speed of your fastest land training - 60-100% at max - and all your mounts that fly now go the speed of your fastest air training - 150-310% at max regardless of what their original limiters were set at. So, grab your favourite mount and ride free :)