$550 might seem like a lot, especially since the springs can be purchased for about $30 each. But the labor and related parts, (hangers, bushings, bolts, etc) probably makes up the difference. It would be nice if they could install wet bolts or better bushings; or at least grease the bolts going into the spring eyes.
Broken & weak springs are not that un-common, especially on heavier rigs such as you have. Maybe you could sweet talk them into installing 7,000# axles while they're at it if yours currently has 6,000# axles.
Good luck.