the cost implications of bringing in new customers as opposed to retaining existing customers?

Answer 1

I think it must be about at least 4 to 1 - the cost of new versus retaining old.
Retaining old should be easy - if you provide good service at a fair price - but getting new requires a lot of marketing expense and often "loss leader" incentives resulting in lower profits.
I would always go for retention of customers where possible.