Beware of assuming there weren't many carbine stocks lying around. When the "long" 1899 carbine stock was adopted, there were some 25,000 earlier carbines on hand, all of which were slated to get new stocks and sights. This would, of course, have required making about 25,000 extra 1899-style carbine stocks. As collectors know, many of the earlier carbines survived with their original short stocks - probably because the adoption of the M1903 made it relatively pointless to continue spending time and money updating obsolete carbines. This may well have left a very substantial pile of carbine stocks in storage.

As for government avarice - you might want to consider the price for normal rifles fell in 1926 (from $6.00) to $1.50 and those cut off to carbine length were $3.50, including a new '03 front sight assembly and a new carbine stock. This was significantly below either original cost or contemporary commercial surplus prices.