I'm guessing that a simple phone call could shed light on the situation.

If they want to play rough, they have the power to fine and deport you for trying to enter China without a visa. They can also be nice if they want and arrange your return to HK without a fine. Point is, it's an administrative hassle for them to get you back to HK, and you should not show up at Luohu unless you're prepared to pay the fee, whatever it is.

There are certainly examples of people leaving HK, being refused entry to China, and returning to HK with no great extra hassle. It happened recently to someone who had taken part in the Occupy protests (but who had not been arrested or charged with anything) - she tried to enter China at Ching Ming with her family to visit an ancestor's grave and was told at the Chinese border that her entry permit had been withdrawn (unknown to the HK authorities).

Apart from the phone call idea, all it would take to confirm the current fee structure, would be for a single one of us to check out details on their next pass through a relevant checkpoint. Failing this sort of DD, be prepared to pay up if you don't want a headache.