I haven't looked in about a year or so but South Lake Union itself has some very nice brand new class A and more under construction. Prices when I looked were in the neighborhood of $20-22/sf but most of those spaces were too large for a startup (they are looking to rent out a floor at a time). There are some older buildings you can find for $18/sf. When I looked I could not find a space small enough for our business but at this point they may be willing to divide as well as drop the price.
For small groups there are also cheaper options such as renting a desk from Office Nomads which runs for $475 and includes Internet.