I think going into messaging at all would be extremely risky and you'd need to have a really fantastic product and a clever business plan to make it profitable. The space is already dominated by free options - all of which are funded by giant companies that make their revenue on other products.

The only paid client I know of is Trillian Pro. They've been around for quite a while but I have a feeling it's a small company with low overhead. It would be interesting to know what percentage of their revenue comes from the paid pro version vs the ad-supported free version.