Batgung

I suggest you get in touch with an estate agent in those areas (wherever Tuen Long is I don't know). Lam Tsuen has a couple of "Oriental Property" agencies (they advertise a lot on gohome) - one at the entrance to Fong Ma Po (where the wishing tree is) and the other at the entrance to Tong Sheung Tsuen opposite San Tsuen. They have English speaking staff if needed. Tai Po has village land around Wun Yiu, San Uk Ka and also lots out along the Ting Kok Road (strictly speaking Lam Tsuen is in Tai Po district anyway)

Your question is difficult to answer because any available land that falls into your requirement will be owned by one or more villagers (the latter given the size you are after) - and they can charge whatever they want assuming they can agree on a price between themselves. Forget about there being any market rate.

Not sure how it works now, but when I first came to HK owning land wasn't enough, you also had to obtain the 'right to build', which was issued to qualifying indiginous male villagers by the govt.

Unless you had both, i.e. land AND build rights, you couldn't do diddly squat and thus lots of money changed hands as those with RtB but no land, either bought land or sold their build rights etc.