The cost for veterniarian proceedures varies greatly depending on many factors:
- Locality (roughly, services provided in cities are more expensive than rural services)
- Veterniarian speciality/experience (if a general vet can perform the services it'll be cheaper than an extensive procedure that requires a dental specialist)
- Surcharges for special situations. Some breeds require extra care during anethesia due to sensitivity or size. Medications may need to be adjusted if your pet is currently on other medications.
- Types of procedures. Most veternians have standard charges for regular teeth cleaning and extraction, but extra services will cost more if they're necessary. For example, one of my cats had a growth on his gum line that we sent off for biposy to be sure it wasn't cancer, and that cost extra.
If the problems that you are concerned about are that the dogs will need cleaning and minimal tooth extraction, I would suggest calling your veternarian (or the vet you're considering, if you do not currently have any pets) and asking directly for their standard charges for dental cleanings.
If the dogs have considerably more complicated dental problems, you can count on a cost that is several times the standard cleaning. The shelter/rescue may be able to get (or may already have) an estimate (or at least a ballpark), depending on their agreements with the veternarians that they work with.
Keep in mind that many of the proceedures performed with human teeth (such as filling cavities, root canals, etc) are not normally performed on pets, instead the tooth would be pulled. I don't know how a dog experiences loss of more than an occassional tooth, but I have a cat who had such severe dental problems that we had to pull all of his teeth, and he's fine (is mouth is healthier and less painful now). He's even able to eat dry food (a bit messier now, but fine).