For repeat customer performance, Profit from repeat customers is the best metric to watch.
Being based on profit, it's a core business metric. Direct costs to acquire repeat customers will tend to be lower than new customers. Indirect costs could be higher as repeat customers could use a greater portion of overhead costs as your loyalty, retention, and other programs are long-lived before you see a repeat order.
You'll know the process is working well as long as it's positive, trending up, and a good enough business return.
If costs are difficult to separate and calculate for repeat customers, revenue from repeat customers is a good alternative. As long as you watch your expenses and margin, segmenting your revenue into new/repeat customer can show you the health of your business as a whole.
Eric Davis
Measure your customer loyalty
Measure the different levels of customer loyalty with Repeat Customer Insights. It uses various models to segment and grade your customers based on their behavior.