Average rent for a studio/efficiency during summer season?

I would check the rentals section of this seattle home values page: http://www.zillow.com/seattle-wa/home-values/

This map might help, while it is for 1-bedrooms, it will give you an idea of which neighborhoods are more affordable: https://www.zumper.com/blog/2015/04/see-how-much-seattle-spent-on-rent-this-march/

You don't mention a neighborhood, but unless you're in a really unpopular neighborhood or renting a microstudio, at LEAST $900 / month before utilities. Microstudios will run you closer to $700 - $800, depending on neighborhood and company.