To give a customer different prices based on quantity, we add what is called Tiered Pricing.
To give your product a Tier Price, navigate to Catalog > Manage Products and select the product to edit.
In the Prices tab, you'll find the Tier Price field.
Click Add Tier.
You'll see a row come up asking for customer group, quantity, and price.
You can set these accordingly, with the specified quantity number being the starting point of the specified price.
Note that you can also set this based on group.
Here are the tiered prices with from the frontend, from a customer who is not in the Retailer group.
And here they are as a customer in the Retailer group.