The cost for each prospect you converted into a paying customer (or lead).  The formula for this is total cost divided by the total number (#) of sales (or leads).

For example; you might have created a Pay-Per-Click campaign that costs you $100 per day and brings in 10 sales or leads per day. This would bring your cost per aquisition (CPA) to $10 ($10 for each sale/lead).

Each business is different and you should first understand how much each sale or lead is worth to you. Determining this will help you set a good CPA limit resulting in a better Return-On-Investment (ROI).