Where do you live? Speaking from experience here...I think your best option is to call a dentist and ask if they will take a payment plan. Explain your situation, I am sure they will be very understanding. I live in Toronto, Ontario and a typical filling here costs around $150.
You could go to a dental school, however it's a lot longer of a process. You have to go in and sign up, take x-rays, and then come back.
I completely understand where you are coming from and have been in your situation in the past. Take my advise that what's $150 now can turn into a $2000 problem later...after a root canal and a crown!
If the pain is really bad, go to a walk-in doctor clinic, or your family physician and explain your situation...they will prescribe you something, probably tylenol 2 & a antibiotic to help out until you can do something about it. Be careful not to wait too long as infections can spread into your bloodstream and become dangerous...this once happened to me and half my face swelled up...scary!!! I had to go to the ER!
I find it so unfortunate that teeth are the only part of our body that can't repair itself, and yet the government does not help us pay for dental fees....
Good luck to you! I know the pain of a toothache and it sucks...
One last word of advise...whatever you do, do not crush aspirin and put it on the tooth...you will burn your gums

I read this somewhere and tried it...not smart. The best thing for pain is advil migrain...this is what my dentist told me.
okay that's it! Good luck!