There are a number of factors that influence the value of properties in Mountain Park; which complex the unit is in, the year built, the features of the unit and development, the condition of the property, square footage, etc.
My office is in Lake Oswego and I live and specialize in the area. If it helps, I can shoot you some information that will answer your questions - no obligation, of course. The 2007 median sale price in Lake Oswego finished up 5.2% over 2006 - compared to 1.9% for the general Portland area.
We've sold a number of properties in Mountain Park - it's a very desirable area.
Best of luck!