Because there were few of these cars built, there will be little comparative information available. There are lists of auction results you could consult, both on the internet and in classic/old sports car magazines published in England & available in large domestic bookstores. Another contact is LMB racing in Belgium who are Lagonda specialists. They would be worth an enquiry.
The big determiners of value are the same for any car of this type: low miles, fine original condition, completeness of orignal factory equipment. Proper operating condition of all systems is important but can begin to pale if the car is sufficiently unique and presentable.
If it's a Rapide it is sufficiently unique.
There's an auction report of such a car in the March Sports Car Market magazine. [color:\\"black\\"]
