Actually, the Mossberg sights are very good. They are not pretty, but they are tough and functional. You could mount a MMC or LPA rear sight, but at least one extra hole will have to be drilled and tapped. Figure on about $120 to $160 for the rear sight and labor.
The front sight can be quite expensive. If you want a clamp on MMC, you can get away for less then $75. If you want a silver soldered blade type you are looking at about $125. It would be more cost effective to buy a new barrel with the front sight already on it.
When people call us to have this kind of work done we usually tell them to sell their old gun and buy a new one with ghost ring sights already on it. You will be money ahead.
I just noticed that you have a 500. My advice is to forget it and get a new 590-A1 with ghostring sights.
