I'll be honest with you OP, this is a tough situation.
Your Ex will know this collection has a ton of sentimental value to you and will overvalue it.
You'll want to keep the value low, so you'll be inclined to lowball it.
The fact that the Magic community is pretty incestuous makes valuation difficult, not to mention the extremely high variance of card prices (most cards can vary by 20-30% depending on where and when you buy them), and this doesn't even factor in trying to assess the condition of each card. Your lawyers are already salivating over the possibility of having to jointly value this collection.
Short of getting both lawyers to agree to maybe contact owners of two major companies (like SCG or CB) and try to get a true bid/value on the collection, I don't see you having a ton of options.
If I'm being honest, I think you might have to consider the possibility of not fighting for the collection. Unless you guys are super amicable, your Ex will know this is a place that they can really hurt you, so I'm guessing will not be inclined to be reasonable. If you insist on keeping the collection, you'll likely have to significantly overpay to do so. You might be better off declining to keep it and preferring a more liquid asset and rebuy your collection. Or try to jointly sell it to someone that will let you rebuy it (though I am guessing this would be a red flag for the lawyers).
I'm not saying this to be harsh, but I've witnessed emotional business settlements before and it's really tough to negotiate for something you can't let go of.
If nothing else, be careful with the lawyers. You can quickly lose the entire value of your collection, simply fighting for your collection. I've seen people spend tens of thousands of dollars, fighting over replaceable items worth hundreds. Fighting things on principal has a huge price during a divorce and a few months later the satisfaction will be gone. This is not hyperbole. As long as you are fighting, the meter is running.