The picture you posted is not very clear and the information provided are not sufficient to provide you with an adequate diagnosis.However, here is what I tell you based on what I think I see on your picture.It appears that you upper right canine (eye tooth) has a lesion at the gum line.A clinical and exam and x-ray are needed to confirmed the diagnosis.This could be tooth decay (dental caries) at the neck of the tooth. Depending on the extent of the decay, the treatment will be do restore the tooth. This can be a simple as putting in a tooth colored filling or it can get more involved if the decay is deep and involving the nerve of the tooth.I would recommend seeing a dentist in your area as soon as possible. Your remaining teeth appear to be in good shape and it is always better to intervene early before the damages are too extensive. As for costs, the depends on where you are. However, if this turns out to be a simple cavity that only requires a filling, your are looking at treatment costs in the 200-500 range.
