How much does it cost to buy and sell bitcoins? I'm a newbie. 

Lets say I see the btc usd exchange graphs and see that it has tripled in the last month and I decide to put $5k in hoping it will grow. 

What is the least amount of USD I could spend to buy $5k and sell $5k on a moment's notice whenever I feel like it?
Mt. Gox takes a 0.6% fee.

Bitstamp a 0.5% fee.

You also have to consider the time it would take you to get your account approved, and your cash to reach the exchange from your bank, plus any bank fees (both sides).