R12 is coming down in price because more and more cars requiring it are leaving the road every day. The last pounds I bought were $29 each. I expect it has come down in price since then, especially in the Winter. There are many problems associated with replacements such as Freeze 12.
I have even reverse converted one of my vehicles BACK to R12.
R12 is the least expensive component of an a/c repair. Stick with the R12 and you will be glad you did.
If I were to convert any more cars, the ONLY choice I would make is R134a.