It's because a double action revolver requires more complex machining and hand-fitting than a semi-auto does and is thus more expensive to produce. Also, revolver prices had been depressed since the 1980's because when police switched from revolvers to autos they flooded the used market with trade-in revolvers at bargain prices. Revolver manufacturers had to compete not only with each other, but with their own products on the used market.
Within the last ten years or so, however, the police trade-in revolvers have pretty much dried up and so revolver prices, both new and used, have steadily climbed. On the other hand, prices for semi-autos have come down, polymer in particular. This is because more makers have entered the market so the competition has driven the price down. Also, we're beginning to see police trade-in semi-autos come on to the market which depresses the price in the same way it did revolvers.
