How does lead work?

[Kimberly T]

Mortgage lead or apartment lead, or home
for sale lead..

How much is it worth, if it leads to a referral that ends with transaction?
How much work needed in terms of time or cash?
I am asking: how much pure profit should be expected per referral that ends with transaction?

For example someone says I am looking for an apartment. I show an apartment that meets criteria. It ends with a lease. It may also not lead anywhere. In this case it is a loss.

How much do you think a visitor to rent.com brings in cash? $10, or $30, or $100?