It all depends on your take on fantasy, your gm and your gaming style.
In the right environment tigres may be pretty common. With magic they might be pretty easy to train, thus you might get them pretty cheap, as the price stated. This subject has been raised before. A heavy warhorse can be pretty awesome in a fight, a tiger is better.
If tigers are that easy to come by then should they be part of NPC basic equipment? If that's your world mechanics, encounters become somewhat harder. Is the mount the encounter, do you always consider it in the CR? After all, take out the rider, the mount is out of the fight... (A heavy horse is CR2 by itself).
You have to think what you are doing to the game and what consequences that might have. Some things in game don't have to balance power with cost, in many games you simply won't be able to get a trained tiger. In those you can, enjoy, but be prepared for how the gm will balance things.
Also be prepared for if someone controls it and turns it on you, +4 will save is pretty poor...