Of course, if that was the method, then diseases like HIV would be far down on the list instead of it's current place near the top. The more celebrities you have on your disease, the more money you get. This is an old graph (2002 I think), but I think the relative comparisons haven't changed much.
The very widespread diseases are also a HUGE drain on the world's resources in many other ways besides just research money. The money spent treating and managing the top diseases is ENORMOUS. This is all money that isn't available for doing other things in our economy.
But, incident rates still don't tell the whole story. Diabetes (particularly Type 2), for example, is a major contributor to the increase in many other diseases - kidney disease, heart disease, strokes, vascular disease, neuropathy, etc. Cure Diabetes, and you would make a significant impact on all the other diseases where Diabetes is the actual underlying cause.