Post a pic of the intake runners. The B heads will have two, one square one and one round one. C heads will have a single runner. $100 for B heads is not uncommon, you can often get them for free. They are not desirable at all and they are far and away the most common 4V head as well. With that said they make good power and if you can get them cleaned up for a few $hundred and the springs are good, B heads can be a great budget option. Your big expense with B heads is the intake manifold unless you use the Mark VIII one.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.