Richard Nixon has learned a few harsh lessons about the real estate market. He finally sold his Saddle River, N.J., home but got $850,000 less than his original asking price. The former president and his wife, Pat, put the house up for sale in May with a $3.25 million price tag, but they are getting only $2.4 million from a buyer who did not want his identity disclosed. Nonetheless, the Nixons are making a nice profit on the deal since they paid only $1.25 million for the house in 1981. They are moving to a three-bedroom condominium in nearby Park Ridge, which will be easier and less expensive for them to maintain.