We each have our reasons for buying, building resin kits. I would be the last one to deny anyone in the business a fair price.

But, here's how I look at it:
I have become obsessed with the car's lading. I model one town and know what the customers served received and/or shipped. I have several 'lists' of cars that were on the railroad over 15 year or less span. These lists contain the car's lading. I therefore try to buy only cars off these lists with lading for my customers. That means I usually buy resin kits.
Because of the before mentioned criteria I do not bother with any of the national freight car percentages tables. Because I'm 'Micro" modeling.
How do I value my cars? I have a spread sheet with my inventory and base price (kit, trucks, couplers). But, to me the valve comes from assembly time and play time. As I said, I build the models to operate, so I get hours of enjoyment watching the models move on the layout knowing I've got the right car at the right business (well, close as I'll ever get in most cases). As for assembly time, I usually build the cars as they come. I usually don't buy over challenging kits like tanks or hoppers. I did build a SC&F tank car though. I usually slap a car together in 4-6 hours.
That's about all assembly time fun I can stand for my money : )
Clark Propst
Mason City Iowa