Number of horses remains fairly constant (around 25 million) between 1910 and 1920 (only date for which stats are available). The majority of horses are for agricultural rather than domestic use and the cost per acre of mechanised farming is a little higher in 1924 than cost of horse-powered farming. Car and Truck registration in New Mexico in 1923 was 32,000, in 1913 it was only 1898. Total registered motor vehicles in the US in 1923 was 2.5 million. It looks like people were buying the motor vehicles but are still far more reliant on horses.
Even had the number of horses decreased it would simply underline the fact that just because technology has moved on, it doesn't necessarily make things cheaper, there are other factors and comparing such disparate periods without considering the rise of fascism, the formation of the USSR, the introduction of insecticides, formation of FBI, restrictions on immigration and dozens of other changes between the two dates is simplistic and prone to error.
14 Years of change is more than just 14 years of increased mechanisation in motorcar factories, there are hundreds of other non-technological changes that will influence the selling price of a specific vehicle.
The German army in WW2 was one of the most technologically advanced armies in the period. They were still heavily reliant on horse drawn vehicles.