"But thats a low end task".
It's also about 90% of the market. Macs have a significant share of this market -- sold to those who care more about design, stability and/or status more than price.
$800 should get you an i5 2500K, a solid motherboard, 16 gigs of RAM, a small SSD, a nice case & PS. No ECC, but it's considerably faster than a Mac Pro.
For the 1500 machine, add a killer graphics card, a secondary HDD and a Windows license. You'll even have some money left over to sub in one of those silly "gamer" cases.
That's BYO. Prebuilt stuff is generally about the same price, but comes with lower quality cases, power supplies & motherboards.