Is $5,000 to fabricate and install a 24ga chimney cap a fair price?

I have a relatively new home (less than 5 years old) that developed a leak in the ceiling of the living room, which has a vaulted ceiling and a gas fireplace. Initially, I thought the leak was from the chimney-roof join, and a roofer tidied up the flashing and shingles, but the leak returned. It was then determined the leak was from the vent at the top of the chimney. A chimney guy applied sealant, and it hasn't leaked since. However, my original builder suggested a more permanent solution, specifically a fabricated chimney cap. I should note that this is a tall, steep roof, and I cannot DIY this work as it requires a lift.