You certainly could, but I see no problem with defining a single global variable. It's not like you'll be generating realNumber ten times in the same page and calling it might get confusing. Plus, if you end up re-declaring it somewhere, it won't really affect anything else, I don't think. Especially when the function that declares it originally is only triggered on a click. Also, I apparently hadn't quite understood your math logic, so... the variable definition would look like:
realNumber = (Math.ceil(Math.random()*(Highest+LowestButton)));
Instead of the one I gave you earlier. Once again, global variables aren't all that evil and scary, and returning a value from a function that isn't something incredibly long and complicated shouldn't really be needed.