There is no set ratio of land to building - at least not in BR as far as assessments go. The assessors office divides the town into about 200 distinct neighborhoods...an acre in one place might be valued for tax purposes very differently than an acre across town.
I have found that the people in the assessors office are very easy to talk to and open with information. Your best bet would be to call them directly.
All home sales ae public records. You can use the following link to look up any Land Records & Deeds in NJ by county or zip. Unfortunately, agent information is not available. Another source is a Local Weekly Newspaper as I see others have suggested.