Steve,
You must get the dent raised before using the gun again.
If you fail to do so you stand a high risk of a ring bulge. This in turn can loosen your ribs at that point.
Afraid I can't help you on cost as it depends on who you can access but in the UK you would be looking at about 50-60 to raise dents and a quick polish.
It MAY be safe to use but you may further damage the barrel in the process. Plenty of guns have been shot with dents in the barrels and the shooter has survived to shoot aniother day and lots of barrel-smiths have made a good living fixing the damage!
