I sure hope that's $70 for the bobbin case, not a bobbin. Even for an original bobbin case that's high.
People like Glenn Williams in Florida sells them for under $50.
http://pages.suddenlink.net/joyof301s/glenn.htm
Additionally, a well tested replacement case works properly as well. For instance, Sew-Classic sells tested ones and offers a satisfaction guarantee:
http://shop.sew-classic.com/Bobbin-C...-SCBC45751.htm
I've bought from both and have nothing but good experiences with them.