I doubt if bias will be cheaper but I would much prefer bias. Radial offers three qualities each not needed with a trailer. They ride smoother, You do not need this. They get better traction. Again you do not need this because if it is a tandem they need to slip sideways in a bend, In fact a radial can grip the pavement so well that it can loosen from the rim. They wear better. Wear is not a consideration for a trailer tire used infrequently, Also a bias has a stronger sidewall.
I would pay a premium for bias.