Yes, it's very important. $400 seems very reasonable, too.
I know my vets have always recommended daily brushing for the dogs, but I'm lucky enough to get my teeth brush and the kiddo's teeth brushed appropriately. And I have 3 dogs so it would be quite the production. But I strive to do better. Also those greenie chews can help keep plaque at bay.
Basically, if you don't do a lot of teeth maintenance, the dogs can end up needing extractions and have dental decay that spreads. And dental health has been linked to cardiovascular health in humans; I seriously doubt dogs are any different.
So, yep, get it done, buy them a toothbrush and toothpaste too, and do the best you can.