@Greg:  You've put your finger on the problem with market value--it depends entirely on finding a buyer who will pay that much.  I know of some rural land around here that sold for over $5500 an acre back when the market price for similar land was about $3000--because the buyer wanted that exact piece of land.  Was that what the land was worth?  Arguably it was--someone paid that much for it.  But any other owners who began to imagine that their land was worth as much ended up sorely disappointed.

@Guest:  The productive and rental value may be difficult to forecast into the future, but surely it's no more difficult to find out what similar houses rent for than it is to find out what they sell for.  You can get a pretty good idea just by checking the "for rent" ads in the paper.

The thing about rentals is that the information is current: The amount of the rent gets reset every year, and the rent actually gets paid every month.  On the other hand, a house can go years--even decades--without being sold.

Unless you have an offer in hand, the market value is just a guess.

There are plenty of people who will make that guess, based on recent sales of similar properties (and other things).  But I think homeowners who take that guess, subtract what the owe on their mortgage, and then call that their "equity" are just fooling themselves.