I used Hertz. I have a company rate code (whatever they call it). Like I said, I picked up at a downtown Seattle office and returned to one in downtown Vancouver (somewhere off of Granville Street I think). The cost was US$116.80 (just verified, reserved a C/mid size, they gave me F/full), plus gas. I drove 149 miles, @ US$.38/mile = US$56.62. If you rent from an airport, there could be additional fees.
Most US insurance policies work in Canada, so I didn't need that, but you might want to review your policy first.

Generally for one-way rentals, you have to pay mileage/kms, so it could cost a little more.

If you haven't yet, I'd check out the Hertz forum here on FT (and possibly other car rental forums if you aren't tied to one):
http://www.flyertalk.com/forum/hertz-423/
In the Hertz forum, there's a thread about coupon codes. Just plug away until you get the best rate. My company code nearly always works best for me.
If you haven't yet signed up for a car rental program, I'd highly suggest doing so. Along with earning points (this is Flyertalk after all), you can usually avoid the lines at airports and go straight to your car.

Of course if you go this route, don't get gas in Canada, where it is more expensive. The cheapest stations (Sam's or Costco) won't be open at your early hour, but nearly any station in Washington (other than in right at the border) will be less than in BC. We went to Fred Meyer in Bellingham, got some soda, water, and snacks, and filled up there.
But do top off before you drop off and save the receipt, just in case.

I saw something on one forum (maybe it was Hertz?) about Canadians driving rental cars across the border, but it sounds like whatever issue was going on has since been resolved.
I had absolutely no problems driving my rental across. Actually the agent didn't even mention the car. I showed him my passport card, he asked where I was from, how long I'd be in Canada, when I was last there, where I was staying, etc.