Andrew,
Matt and Larry each have great points...
1. The only way to truly determine value is to have a Realtor or an appraiser assess the value with analyzing comparable homes. Many homebuyers can use http://www.Zillow.com or http://www.cyberhomes.com, but these values are by no means accurate, more to help give you a value range. It is the current market valuation that narrows the price.
2. Tax assessed values are no longer more than current values in King County unless it has been adjusted. I also find the prices are about 25% less than the tax assessed value, but each property is unique. For example, some homes value is in the land, but the market for land is so slow it is hard to justify it in today's market.
Let me know if you are interested in knowing a home's value. I am a Realtor in the area and would love to assist.
Thanks!
Kenzie

Interesting that folks in TX and NC know about Fircrest's tax values! Andrew, that used to be the case, but is no longer valid. So many homeowners have petitioned Pierce County in the past few years to have their assessed values lowered, while other values have continued to rise (while the home value declined) that there's not really any connection anymore.
I've seen some houses sell for 20% or more of assessed, and many houses selling at 25% below value. If there are any specific homes you'd like to get an idea of comparable sales, feel free to let me know or use my sold homes search at YourSoundHome.com

Andrew,
To get out our crystal ball is not a good way to predict. With real estate being so localized your best bet is to get a local Realtor to pull the comparables and average the price that way. A Realtor would look at anywhere from 3 to 5 homes that are actively for sale and the same for ones that have sold. Now the homes would have to be of the same approximate age, size, #of rooms, in the same area, and other features. We always adjust the other properties values not the subject (property in question). For instance if the subject property had 3 full bathrooms but one of the comparables had only 2.5 baths then you would add to the value of the comparable to bring it up to the same standards as the subject.

Either way if done properly it is not a quick and easy task. So contact a good buyers agent in the area you are considering and have them do a comparable market value on the home you are considering. If you need a referral to a top agent in your area feel free to contact me via my profile here on Trulia and I can put you in contact with one of our top agents there.

Hope this helps,

GREAT QUESTION:

You are pretty much on track