She probably has some buildup that won't come off causing that stinky breath. I'd let your vet check it out and see what they say to be sure though.

Cost of a dental depends on your vet but it is impossible to get off the calculus/tartar that is built up on the teeth yourself. It really is like cement almost, very very hard. I see lots of bad teeth/mouths at work and they can be very very bad for your pet. At my job it's around 100 dollars, little more if they are older and need pre-anesthsia bloodwork but it's well worth it.

Oh and if you got it done, then if you brush her teeth a few times a week, that will help to keep the buildup down.