land is not cheap in this area. But I did see a few vacant sloped lots for sale 10000sqft or less.
In general Tigard/Tualatin/ Beaverton is a well developed suburb and .5+ acre lots are far and few between. These suburbs create the boundary for Metros Urban growth boundary and so I feel that creates some artificial inflation that is not going to change anytime soon.
I currently rent a 2acre little house (the land lord is a tough cookie who will be holding out unti the bitter end) and it is tax assessed as is for 300,000. Presumably she would raise her costs upon spliting the lot for probably close to $200k a peice. but realistically she will divide it into 4-7 lots to an investor for top dollar.