I have to agree - it's rarely an issue of what can "I" afford for customer acquisition, as much as "what is the industry norm for customer acquisition." With several million orders per year going to FTD.com, not to mention their top competitors, the norm is set by them - not us. This is the ballpark, and we can play or go home.
A few years ago FTD disclosed that they had a customer acquisition cost of $23 for customers via PPC advertising. They determined it to be worthwhile since they know the lifetime value of a customer, and they do a good job of retaining customers via email marketing, etc.
The question isn't whether or not we can afford to pay $15-20 for a customer. The question is one of scale: how many customers can we afford to acquire in a given period. I've seen florists do too well with their marketing and not have the bankroll to sustain the growth while waiting for the longterm value. Know your business, know your finances, have a budget and plan it well. Work on retaining customers, not just getting them.
If you can master the art of retaining customer and getting them to order more often, you can afford to outspend the competition on a per customer basis.