That's exactly what this is.

You've invested time or money into something, there's no sign of it paying off in any way, but you should continue on anyways because otherwise you are wasting the resources you've put in. And maybe, despite all indications, you just need to invest a little bit more for it to all pay off.

It applies as well to a gambler sitting at a slot machine, or buying lotto tickets as anything else.

Not that it would really work in the comic... but it seems like it could be improved if it was made clear that the guy on the bottom had been gradually finding more and more diamonds as he went, just not the jackpot he was hoping for.

Then maybe the message could be to not lose hope if you are showing improvement, just not at the pace you hoped for.