It depends. You can find games at every prices. The lowest price for a Steam game is 95 coins, the highest price is 40000 coins (for this price, it's not a game but a collection, like the "THQ Collection").
Generally, the admins buy the games when they are on sale. More the game is on sale, more the price is low. So if you see a game with a very high price, it's better to wait a sale!
When there is a bundle like a Humble Bundle, you can have a lot of games for few coins (For example: Dead Space is only at 695 coins).
For the gift cards, it also depends. You have a Steam wallet code at 5$ for 4500 coins to 50$ for 42500 coins. Amazon gift card at 5$ for 4500 coins to 50$ at 41000 coins (€ and £ have a different price).
As you can see, it's very difficult to explain case by case!