Well, if you are in game, you can check them yourself. Hard to tell - what is useful to one person is not to others. Some things are common, like 19-dia puri, but otherwise, usefulness in eye of the beholder especially for new player.
Mount prices are 200-400 dias. 2 seaters are more expensive, flying mounts (to cross water) also add to cost. Most mounts are 65% speedup, some are 70% (flame/star popular choices for those), and one (rune disk) is 75%. There is a 6-seater mount, rarely on sale, that is like 600 dias.
Until you know what class you want and sure the toon is permanent, I would advise against mount buying. Instead buy horse tickets in AH. They are cheap and generally useful. Only once you are set that this is a toon you want to keep playing, you should look for mounts then.
And, yes, there are daily, weekly and weekend sales. Mounts are rarely included, but if you are patient, there are deals to be had. My Flame mount was 163 dias (regular 399), my alt is on a Unicorn for 99 dias (regular 199), etc.
-- Rustyx --- 92R / 92S / 92M on Reni (Guild KnightShift). Yes, running the new FOTM R/M, cause I am not elf enough to be WD/S.
Oh, and people who have more than 3 classes are clinically insane.