A new cellar bulkhead door costs (door purchase price) about $500-1000 depending on whether it is plastic or fiberglass, or steel. Removal and disposal of old one and installation of new one could run from a low of about $400-600 if the fit is basically pefect, to around $1000 if a bit of concrete chipping or buildup is necessary. Hence, total replacement cost around $1000-2000.
Unless the rust is really bad, you could probably get a painting contractor or a good handyman to remove it, sandblast, metal prime, and double coat paint it and reinstall and trim and caulk around it for about $600-800.
If the rust is only very local and not widespread, then $300-500 is probably good for power wire brushing or naval jelly rust fixation, overall wash to remove dirt and oil, rusty metal painting with a fishoil based rusty metal primer like Rustoleum and PPG make, double coat paint entire unit, and caulk and seal openings.
As long as the doors fit fairly tight, caulking and installing trim and weatherstripping to keep out rodents should not be a problem.
