The thing with vaccines is it costs a lot to do the R&D, all the required tests/clinical trials, approval process, etc. Companies that make vaccines are a business. They are doing it for profit. You don't make a vaccine for a disease that nobody has, because you lose a lot of money.
With SARS, China didn't report the disease till Feb 2003, and by July WHO had already declared the outbreak contained. Since then there had only been a few sporadic cases, mainly by people working in labs and such, with the last case in 2004. A vaccine would not have been ready in the short amount of time the SARS outbreak went on for, and they aren't going to keep pouring money into it when the disease isn't around any more.
Likewise with MERS, there are only a couple hundred cases a year, and virtually all of them are in the middle east. It doesn't make financial sense for companies to make vaccines for it.
I wouldn't worry about the vaccine being expensive. Economies of scale and all that. Countries would likely foot most of, if not all, the costs anyway because they want their economies to go back to normal.
You are right in that we don't really know what a vaccine for corona will look like. It could be perfect, or it could not work well at all. All I am saying is the lack of vaccines for SARS or MERS shouldn't be a cause of concern.
