I'm gonna disagree with the 5x as expensive number to attract a new customer, at least in my experience. That may have rung true in the past, but once you build a good SEO based SalesFunnel, the cost of running it and staying on page one in search, where new customers find you, is not that great.
In my business, I receive new leads from people who found me on a generic Google search every week. And they cost me $0.00 - although not all of them end up qualifying as new customers, those who do still cost me nothing and are worth anywhere between a few thousand dollars per project to 6 figures over the lifespan of the relationship.
Keeping an existing customer engaged will cost you a significant amount of time and attention and sometimes money - especially if you need to rectify a mistake that happened. And it doesn't matter if that mistake was your fault or not.
However, the investment is well worth it and will pay off multiple times what you invest. I have customers that are so loyal, that a competitor can come in and underbid me by half, and the customer will not switch, because of the relationship we have build.
