In good condition, $400 would be a smoking deal for you. But 50% finish sounds like a pretty beat up gun. If it is a project gun to rebuild, $400 might actually be a little much to get back any investment. What do the internals look like? If this is just a refinish, you can add about $120-150 for your total investment. $550 for a rejuvenated Para is a fair deal, but again this doesn't include any r&r of internal parts.