Look at a Huglu SBS. These are very decent SBS's, though start at about $150 more than your $500 limit. Scrounge gun shows. Often the Savage Fox Model B's in decent shape can be had for around $500. These are not 'great' SBS's but durable and dependable. Anything in 28 ga. will be pricier than 12 or 20 I would guess.