The 2017 10-K lists it as:
"Marketing, Promotional and Advertising Costs
Marketing, promotional and advertising costs are expensed as incurred and are included as an element of selling, general and administrative expense in the consolidated statement of operations. We incurred marketing, promotional and advertising costs of $66.5 million, $48.0 million and $58.3 million in the years ended December 31, 2017, 2016 and 2015, respectively."
With about 100k deliveries that's about $665 per unit.
I am assuming that the bulk of this cost is the referral program.