Is he buying from the factory or a new owner?
In case if it's a new owner -- when the owner takes it out of the factory perimeters -- the value will drop some immediately (in the order of 8~10%). Maybe about $12,000 then depreciated?
A brand new one (Spirit deluxe) from the factory is about $13,000 I believe (loaded with all options). Depends what the options were, the owner should have a price sheet for the options and you have to add up option pricing and deduct 8~10% from the total trailer value I guess. Casitas come in many different sizes and different option levels in each size.
2002 value is the latest you'll get. Go into nadaguides values for casita
(2002 values below) price the options and then add half the yearly depreciation (between 2001 and 2002 values) for a specific type of 2002 casita
to the price, you'll get your mid-2003 used price. But then that's what is book value. If it's not mint -- it's a whole other story.