Doesn't DME work out to be cheaper per gravity point because there's no water in it?
That's how it has always worked out for me. But I understand pricing varies widely.
I use LOTS of DME and by far the most economical purchase I can make is a 50lb bag of DME from Northern Brewer. But I live in the mid-west, so shipping isn't too bad.
Shipping for LME is also more per gravity point...
Elsewhere, I'm sure you want to get it closer to home.
I try to buy at the LHBS, but $1 or more per lb of extract adds up.