Deductibility of a Modified Wheelchair Van

I have a client who purchased a modified wheelchair van to transport his wife who is under memory care assistance at home. The taxpayer tried 3 difference memory care facilities in 2025 of which he tells me were traumatic for his wife and he decided to keep her at home with in-home care givers. He needed a vehicle to transport his wife to and from doctor's appointments and to just transport her in her wheelchair. In December 2025 he purchased a 2015 Dodge Grand Caravan for $36K which had only 16,000 miles and already modified for wheelchair capability. The IRS allows for a deduction for the wheelchair-accessible van for the price difference of the cost of a standard van vs the medically modified van. The issue is there are not many 2015 Dodge Caravan's around to determine what the cost difference. The client says he knows cars and no one would have paid more than $7K - $10K for such a 2015 standard van. I searched Kelly Blue Book and other sources and the trade in value for a like van with over 100K in miles ranges from $5-$24K. What other sources may I use to determine a cost of a standard Dodge Grand Caravan without the wheelchair modifications to determine the deductible amount? Has anyone had a similar situation and how did they determine the deductible amount? Feedback will be greatly appreciated. Thank You!