The front sight issue seems to hit about 10% or so of the factory 930 SPX models...like the man said, they usually have a fairly significant degree of cant to the left (when viewed from the butt of the rifle). Some of them require replacement of the barrel or removal and proper installation of the front sight. It has been Mossberg's policy so far jusst to replace the faulty barrels with new that have non-canted front sight posts.
Brand new in box models usually run around $600 (may or may not have the canted sight). Used ones in like new to excellent condition don't usually run much less because of the difficulty to find these shotguns, although that difficulty has improved somewhat. If it is in the shape you say it is, I'd jump on a $500 SPX like a fat guy on barbeque!!!