I hesitate to call this an estimate, or even an informed guess, but perhaps by putting down some ideas I might help your thinking.
Ignoring any particular platform specifics I see the fundamental difference of Web v Native is that in the Web case you have some extra work to do:
a). You have presentation separated from the backend, with probably HTTP in between. So there are services to write and issues concerned with notifications and/or polling to deal with.
b). You're likely using a browser to do the presentation so you will be using some JavaScript kind of library, and these are still hard to drive.
So, if you are targeting a single device my guess is that writing native code is likely to be 50% quicker than web-based code - assuming that you really are targetting identical function.
What seems to happend is that we compromise Web Apps, reducing functionality somewhat and hence the overhead is reduced.
Also there's another factor, the UI portions of the Web App are in principle portable, hence in fact the Web App development may be a reasonable investment even if it does take more effort.
I suspect that portability may well end up being a dominant consideration.