Actually, the paladin / shadowknight level 20 horse does give a bit of a run speed bonus. Let me dig up the post that Moorgard states this. (Will edit when I find it.)
Edit:
Found the
post by Moorgard on the main boards.
As for the horse prices... Yes, they run from like 4 platinum on, being faster the more you pay. You basically buy a whistle that resides in your activatable slots. The higher your guild level, the less the prices are, and the more horses that are available. Also, if you have a higher level guild, I believe you can buy horses for a combined status point / coin, making them much cheaper (coin wise.) A level 30 guild also can purchase Flying Carpets (seen in the Mages Tower in SQ) or Spirit Steeds (seen in the Temple of Life.)
Edit:
Here is the post by a dev about the guild bonuses for higher levels. It has the levels that prices are reduced and the horses that will be available for higher level guilds.
Edited, Wed Jan 19 16:26:00 2005 by Lewskinslayer