Does anyone know the approximate cost to have oral surgery on tooth #31?

It depends on where you go. If you have a University Dental School close by that takes patients, they can be relatively cheap. If you haven't had dental x-rays done in awhile, you could pay for a private dentist anywhere from $125 up for a tooth extraction. Also, there are free or low cost dental services offered through RAM clinics but you have to stand in line for that and have to find one. Check with your local Public Health Department. In the meantime do what I do, go to a local pharmacy and get a temporary filling replacement kit. It is temporary but until you have the money it works.

ritadm,
How do you know it will be a "simple" extraction? If the tooth is falling apart or has decayed so badly there isn't much tooth structure to grap on to with forceps there is a risk of the tooth crumbling when being removed and this changes the type of extraction from simple to complicated. Every dentist has his/her own definition of "simple".
I'd suggest you go to a couple of dentists for estimates. You will have to pay for one x-ray and a specific exam.
Dental schools are a good choice if you have one close by. There is a fee there too but it is normally less than in a dental office.