Depending on your race, you can go to the town before the Big City of your race.
For example, Bloodhoof Village for Tauren and Khanaros for Gnomes. Also, some exceptions like NE mounts being in their capital city, Darnassus, at Cenarion Enclave.
If you got Honored with your Capital City, you will get 10% discount, aka the mount and skill together will cost 90g. If you got Honored with Capital and pvp Rank 3 (Sergeant for Horde and somethingsomething for Alliance) You get -20% discount, equaling mount cost of 80g. BUT, if you just hit rank 3 this week, you need to buy the mount and skill before next weeks tuesday to get the additional -10% discount from Rank 3. Since new honor system will 98% surely come at next weeks tuesday, the discount is no longer valid after that(From what I understood, correct if I'm wrong). If you don't have Honored with Capital, nor Rank 3 right now, it will cost 100g for mount and training.