I agree with Jim 100%. Replacing planks can be tricky, especially if it's a glued together floor. The cost will likely vary some by region. For most repair jobs, if it takes two hours or less an installer is likely to charge a "minimum." In Alaska, minimums range from $100-$200.00. If you go through a retailer you are going to pay whatever the installer's minimum charge is plus the retailer's mark-up.
If the floor is a snap-together floor, you could replace the plank easily yourself. All of the cuts have already been made and you'll be surprised how fast you can get the floor pulled apart and put back together. Be careful not to damage the edges of the good boards while removing them.