who gives a better ROI - new customers or existing ones?

Let's say you got a marketing budget of £1000 to generate more revenue for your bus company.

Ignoring the whole 'brand messaging' thing, how much would you allocate to enticing new customers on to the bus, and how much would you spend up-selling existing ones?

Many marketers in the industry would see new customers as the ROI holy grail and will go the ends of the earth to secure their spend.

But is the investment really worth it? Is spending a large amount of your budget convincing people firstly to use THE bus, then having to sell them YOUR bus (assuming a competitive route) a good idea? If the reality doesn't match up to the promise, they might make just the one journey and you've wasted the spend.

Existing customers, however, have already bought in to the concept that bus travel fits in to their lifestyle. They've already been convinced THE bus is the way forward, and that YOUR bus is the best They now need informing - about new tickets. new routes, new product benefits - in order to generate a larger spend. Simply you have to spend a lot less and have more chance of getting a return.

Whilst I'm not doubting bus travel needs a healthy intake of new customers to help the wheels on the bus go round, I worry about the about of money that is used to target them at the detriment of existing users.