Number of karma votes? Possibly. But then you'd need votes and I'm not sure that is a reliable method.

You don't need to pay out once or at a fixed time. Normal stocks aren't either.

My first idea was that you could 'buy' a meme and advertize so e.g. on your blog. This creates a 'demand' for certain memes. The problem is that you'd be incentiviced to sell your own opinion to cash in.

Then I came up with the idea of 'endorsing' versus 'trading' memes. Endorsing a meme could be taken to be a dividend of the meme. This allows to 'buy' early and cash in on the later endorsing by others. And you could still sell when you have convinced the majority without dropping your endoring. If everybody is only entitled to a limited number of 'endorsed' memes this avoids meme inflation.

The idea is that endorsing is a slow proces you may change your opinion only once a week, e.g. by linkingto it (or its reverse on your blog. But trading can be as fast as you want.