I agree with bilbirk, I did a non-scientific test order one time using Great Models and Squadron. I ordered several items from GM, all listed as "in stock". The prices were slightly lower than Squadron's and if I received everything I ordered, I would have saved about $5-7 total. Their shipping was a little higher, but not by much. Maybe a buck or two.
Of course the order took a while (about a month) and even though I only ordered items listed as "in stock" on the website, I only received about half of the items. I ordered the items I did not receive from Squadron. I received them in about a week.
I did a quick calculation and if I had placed the entire order with Squadron, I would have ended up saving about $6, because GM did not have all the items and I needed and not had to place a second order. Any price differential between Squadron and GM is negated by the fact that you may not get the item. At least when I order from the Squadron site, it lets me know if the item is on hand, or it won't let you order it. Plus the order gets to me in less than half the time it does from GM (which is why I call them "Wait Models").