Welcome to permies, and to the Seattle area!
Finding good, affordable property--with or without a house--is not easy around here! You'll find that most affordable property is either (A) a wetlands or (B) a steep slope. At least, that's what we were encountering when we did our real-estate search 3 years ago. We ended up getting lucky and we have a nice (albeit north-facing slope) 5 acres. It's important to find out how much--if any--of your property is wetlands, because the wetland and surrounding "protection" area are to be left pretty much undisturbed. They make a great Zone 5 area for wildcrafting, etc, but are not too good if they take up so much space that you have no where to build. Our 5 acres is 1/3 wetland, for instance.
I do know that quite a few of my neighbors have been trying to sell their property off and on over the past few years. It seems a lot of people have land they wouldn't mind selling, but don't advertise because they don't want to spend money putting it on real estate listings. Here's a post about one of the properties in my neibrohood that's selling for $118,000:
https://permies.com/t/47577/cascadia/Won-Neighbor. I've walked some of the property and didn't see wetlands...but then, it's 5 acres, and I definitely did not explore it all. It's mostly flat with a slight north-west-facing slope, and seems to get good light if some trees were cleared away. I do not know why no one has purchased it yet, though. It may be they didn't want to live at the end of a gravel road (which my neighbors do a fantastic job of mainlining), they found another property they liked better, they bought a house instead, or something else. I've never met the owner of the property, as he lives somewhere else and bought it as investment property. From area photos, it looks like it has a stream running in the tip of the north-west corner...which probably means there's quite a bit of "wetlands" there. I have no idea how much, though!
There's been at least two other properties for sale around here, by another land owner, with Tax ID numbers of 28070700401600 and 28070700400800. These ones are north-west facing and east facing, respectively. The latter property has a small hunting cabin on it, if I recall correctly, and according to arial photos, it has a stream crossing it from north to south. The first property was up for sale last year for around $75,000 for 5 acres. I don't know how much the second property would be for sale for.
One great resource for researching property is Permit&Planning maps. This one (
http://gis.snoco.org/maps/permits/index.htm), for Snohomish County, shows the elevation of land, where streams and flood zones are, where theres areas of UGAs (Urban Growth Areas, where development will soon be coming). Another great resource is Soil Maps, like this one:
http://casoilresource.lawr.ucdavis.edu/gmap/, and this one:
http://viewer.nationalmap.gov/viewer/. They're a great way of finding out what types of soils a given property has. Another resource is Slope Stability Maps/ Soil Liquidation Maps:
ftp://ww4.dnr.wa.gov/geology/pubs/ofr04-20/ofr2004-20_sheet61_snohomish_liq.pdf. We are prone to earthquakes, and you probably don't want to build on unstable soil that will quiver so much that your house falls down (old river soils wiggle more in an earthquake than bedrock, for example.)
As for areas outside of my neighborhood, there are certain areas you might want to avoid. Granite Falls, for instance, has a big drug problem. So while property is cheaper there, it's not as safe a place. Same with sections of Marysville. We chose to live in our area because there is a lot less drugs and crime, and that was important to us as we wanted to have children (we have one, now

.)