I think your Value of 18K is reasonable and fair for both parties.But that is my opinion here,these things are priced differently in other regions of the country.Easy to understand if your agent feels differently about your Valued price.
I Have been with State Farm for my Chevelles insurance Since they started offering Antique car insurance in 1997.I was with American,but I did not like them.
I submitted an Amount I felt it was worth,with out any Appraisal.They looked at the pictures,and also Agreed with me.Yes it is an Agreed amount,so no Surprises if something happens.Mine does have IL. Antique Plates,and I am Limited to Only Carshows and Parades,etc.No Errands or Joyriding Late at Night.Plus the fact I have everything else in My Life insured with them,means a significant discount.
I Like Them,and my Personal Agent.