You can get a sell now offer from a dealer. This will be less than retail, but gives you an in-hand valuation that you can use. It will be on the low side, but tells you what you can get for the collection right now.
You can also get an estimate from an auction house. This will be a bit higher, but gives you a value you could reasonably expect to get for the collection at near retail within 6 months.
You can also get a replacement valuation, e.g. for insurance purposes, but that will often factor in the cost of acquiring the assets and may overestimate.
Or you could both sit down and agree on a reasonable value.
The advantage of getting an outside opinion/offer is that it reduces future liability if either party were to contest the divorce after it's finalized.