The price really depends on a lot of things, but I think it ends up being more expensive than the previous answers suggested - when a friend of mine had it done recently at a decent clinic in Lisbon, she paid almost 1000 euros for the session with higher quality gel and follow-up care, while at a simpler clinic it runs 400-500. The thing is, teeth whitening isn't standardized: some use LED, others just do polishing, some include actual gum protectors and others don't. If you actually want whitening (and not just cosmetic cleaning), talk to dentists about what type of treatment you need, because that's when the actual price becomes clear.