I just disagreed that a level 1 spell buff of moderate duration should be worth anywhere near 20k, that's just really excessive to me. If you feel the need to jack up the price that much, may as well just ban the item to begin with.
Why is everyone focused on "continuous" items, though? The same equation/multipliers would apply if it were just infinite use activated. If you actually have a choice in the matter, continuous would almost always be the no-brainer pick. So I always figured that in most cases you don't get to pick, the nature of the spell determines which type of item it is and the default leans heavily towards infinite use activation.
So, for the 4000 gp that an above poster tallied, you'd have an item that let you enlarge person yourself whenever you wanted, for 1 min. duration each time.