So, Should the runewords powerful than that? If not, the runeword lost ppl interests or not even the final target. They just become a gimmick. If the answer is yes, however, how much they should price? Should they simply be dropped in game? how rare? how would they affect the gem's price and the legendaries price? That's too complicated for them to decide against to the current D3 system they've implemented.
I think the right answer is that, as far as main stat on armor is concerned, they should fall just below (~50 stat points) set items on the low end, and just be just below the 1.07 crafted items' potential on the high end. You can't add gems to a runeword (its already socketed), so that'd come out to about 250 points on the low end, to about 400 points on the high end. That sounds about right. You don't want to add a whole new tier of items, just give people more choices. What would really set the different items apart, and perhaps make runewords better, is the other affixes you put around that main stat though, like the skill bonuses I suggested.
Oh and I think the drop rate should be low, like 5-10% on mp10 low, so that it's just high enough for you to find one every few games. Also, I always hated the D2 concept of "high runes". In all my years of D2, I only found one, a Lo. I remember letting it sit in my stash for weeks, since it was the only legit high rune I knew I had. A lower but equal drop rate would be much nicer, with runes that you didn't have to combine to make higher ones.