Cox engines went out of production in 2009. 
Of course, keep in mind per capita income was much much lower back then (only $693 in 1941 according to my old Britannica yearbook).
I worked in a neighborhood convenience store in 1974 and there was outrage when a pack of cigarettes went up to $0.20.
Are there still places people go to race bigger slot cars? (about 6 inches) I did that some in the 60s but then everyone got the smaller tracks and cars at home so we did not need to pay to race the bigger cars.
The WWII ones may not be, but the WWI kits are sticks and paper, much like their full scale counterparts.
Again, harking back to the days just before WWII, the best kits were made by Cleveland. These were really beautifully done, and could be used to make a museum quality model (still of balsa stick and tissue paper). Their plans were real works of art. I don’t remember for sure, but I think at that time they retailed for around $2.50. Somewhere around here I’ve got one of these kits for a Messerschmidt ME109, but I’m damned if I can find where I stored it. Probably would be worth hundreds of dollars now.
Guillow’s WWI kits have the basic shape, but the construction is nowhere close to realistic. The vacuum-formed plastic lacks most detail. No provision is made for control surfaces. Yeah, they look like what they’re supposed to represent, but the detail is minimal and nowhere near what you get on even a cheap Bachmann train set.
Model race cars are pretty insane, with limited editions, signed editions, and more.
The sucker still needs to fly with zero rubber band horsepower and suboptimal (read: heavy) balsa. I have a Guillow Halberstadt unbuilt because the lozenge camo in a lightweight format kinda frightens me. I should put in an electric motor.