The "penalty" has mostly been answered in the comments to the question. I'm posting this as an answer as I think you may have misunderstood how try... catch works, given your example and your question "Will the overhead remain throughout the lifetime of the timer t?"
No, the overheard will not remain throughout the lifetime of the timer t, because the try...catch stops being in effect as soon as the timer starts. (well, when the f.start() function returns, which is pretty much the same). The only thing in your code that can trigger the catch is if the timer fails to start (not sure if it's possible, mathworks does not document this sort of things, unfortunately).
In particular, any error in the TimerFcn will not be caught by you try...catch, because the execution of that function is not in the scope of the try...catch (asynchronous execution). If you want to deal with errors that occur while the timer is running you need to implement a callback for t.ErrFcn.