Why do some dentists not offer to pay in installments for treatment?

I have to have 3 large fillings done in a weeks time (my mouth is full of mercury fillings!) and I want to have these white - eventually change them all to white ones. My dentist doesnt offer to pay in installments - its going to cost me �190 for white fillings and �45 for mercury (I just don't want anymore mercury in my mouth!!). We are not covered on my husbands company medical insurance either, only for oral surgery. Can anyone recommend what I should do? Thanks