Apple does have a low-end line it’s called the mini.
I agree with your point about the best price to performance ratio is to purchase a core 2 duo or a phenom2 box and install your favorite flavor of linux.
The Mini is the low end of Apple's line, but with the lowest-cost model being about $600 initial purchase cost, there are a lot of Win-PCs whose initial purchase cost is a few hundred less. By "bottom-feed low-end" I was talking the $300-$400 PCs you see at the big-box stores and such, typically subsidized with crapware.
(Personally I like the Mini a lot hardware-wise -- have two of 'em.)
Apple's low-end line is quite a bit higher than the overall industry's low-end. A $300 system is likely to have an old Celeron or Pentium and Vista Home Basic (IOW, a purposely crippled OS).