The Dragon Knight Saga (DKS) is both games (Ego Draconis and Flames of Vengance) rolled into one, so it's all you need to buy. Only get "Flames of Vengance" if you already have "Ego Draconis".
Prices will vary depending on where you purchase it, but Steam currently has it on special for US$27.
You can't buy it in the shops yet if you live in the US or Canada, but it should be available in most other places. Downladable versions are available on Steam, Amazon and directly from the Larian website.
