Dragon Princes are cool and all, but they are pretty expensive, even for High Elves if I remember correctly. Other than being immune to fire attacks(or having a 2+ save against them, not sure anymore), They will eat some points as you will need 10 of them to be effective.
Just look at the army you are trying to field and see what kind of role they would play. If you feel they are that important to your game plan that you need them, then buy them. Otherwise see if there is something else that can do the same thing for less points.
On a side note, if you are collecting the entire army, you will most likely buy at list one box anyhow. And being in plastic, they should be some good looking models.