Is $3,500 to add additional blown-in fiberglass insulation to the attic a fair price?

My home, built in 1975, has existing blown-in insulation in the attic that has settled. I'm looking to improve comfort and sound control, especially during summers where temperatures routinely exceed 105°F. My attic is well-vented. I'm debating between just adding more insulation or doing a full air seal and re-insulation, as I've read that air sealing might be more beneficial. I'm leaning towards the cheaper option for now.