This came up semi recently. Here they are adjusted for inflation based on CPI: $672k for 1789; $888k for 1873; $1.95M in 1909; $963k in 1949; $1.25M in 1969; and $519k in 2001.

Kind of amazing to see how historically low the presidential salary is these days.

Here are the actual dollar figures I calculated from: http://www.infoplease.com/ipa/A0875856.html

I will let others speak to why.