While Neighborcare is alright for an emergency temp filling, one thing to note is that if it is one of your back teeth having an ache, they'll use amalgam... and they don't do root canals, should that be needed, on molars, only on "easy to access" front ones.
If it's a filling you need, not a root canal, regardless of tooth, I've had INCREDIBLE work done, all composite fillings, at Community Health Snohomish--it's a bit of a journey, but it's SO worth it. I've been to the one at 21st and Yesler, and the wait was 7 hours of me in agony--eating a 21 Rabbits bar cracked a molar from a rather massive hard as a rock seed (maybe a rock)-- after showing up before they even opened the doors. For the emergency thing, Neighborcare might fill it on the spot (if it's needing a filling, that is), but it'll be silver if it's not a front tooth.
UW has a bit of a wait, and they don't do "complicated" services, so it's a little iffy if you're looking at root canals.
Lee Endodontics (this is for root canal stuff on back teeth since the community clinics don't do anything tricky), btw, has a low fee schedule regardless of the person--designed for the mass of uninsured folks in the dental department... their root canals on the far back teeth--the most expensive--are $1065 or were at the end of last year, at least. That is obviously a better deal than the $2500 most spots. If it's a front tooth, most spots can handle them. Lee is up in Shoreline... 13344 1st Ave NE #202, Seattle - (206) 453-349
Community Health of Snohomish is in Lynnwood:
4111 194th St SW # 101, Lynnwood, WA
(425) 835-5204
Having been to both Neighborcare and Comm.Health Snoho, I would never go back to the Georgetown clinic. The dentist didn't even pay attention to the huge print for both a latex AND nickel allergy--I could've wound up in the ER because he was a-ok with proceeding and only after my mouth was full and numb and he was about to start did I ask--wary--if he did composites, which they don't (no clue why unless it's just harder to handle--I HAD insurance then, too! I just needed emergency care and wasn't established with any regular provider yet so I had to hit a community center)...
Oh, and the other warning: if you need a root canal and can't pay, the option is unpleasant (extraction ie yank it out!) Hopefully, even if you have to give up some luxury for a while as I did/do, you can figure out what to skimp on, whether it's a gym membership, beer, cable TV, or artisan bread and cheese (I finally saved up the $1k for the root canal, so I can only say Dr Lee is affordable and Dr Liu recommended him--no review one way or the other--but I'm certainly glad I got the cash figured out since the temporary filling is breaking down bit by bit!)