Quick answer
Cost depends primarily on the collection’s total appraised value and where it’s stored and displayed, but scheduled fine art coverage is generally priced as a modest percentage of insured value annually, with adjustments for security, location, and how often pieces travel to shows or restoration. A well-documented collection with good security typically costs less to insure than an equivalent collection without either.
The only way to get an accurate number is a real appraisal and a full asset review, since the pricing depends entirely on the specifics of what’s being covered.