Misunderstanding
Ok, the DMG says +6 is 72,000. They are talking about having several enchantments that add up to +6. A +5 Flaming weapon, for example would cost 72,000.
The 720,000 that the ELH refers to is the cost for a +6 Enhancement bonus.
They are both correct, they are just refering to two different things.
Edit: Had +4 instead of +5 in my example, oops!