Frankly speaking, I could imagine that by being available to jobs in the city (or at least Kowloon side) she might more than make up for the 5k in rental difference.
How about you at the other side of Tai Po - Tai Po Road, between Sha Tin and Tai Po? Commute for you is the same or less, but the whole of town is suddenly accessible without resorting to driving. There are some very posh estates there, but also a few villages near the university campus (which is one of the nicest places in HK). Granted, no beach, but plenty of beautiful views.