I'm surprised at all the people saying that Nintendo would drop the price of the 3DS as well. They just launched their first system... ever? (not certain) which is selling at a loss from the get-go. The 3DS itself only turned profitable about 6 months ago and has been selling extremely well for quite a while now. I think it would be a very, very, very hard sell for them to make to the investors that they should drop the price again "because Sony is dropping the price of their machine!" Hell, they might need the profits from the 3DS to offset the Wii-U's losses.
OT: I have no idea really. I bought mine for £180 with a copy of LittleBigPlanet, which is £50 less than the RRP. Most of the places I see selling Vita's have the wi-fi model for £200. So I'd say a price drop would have to be lower than either of these. £170 with memory card and possibly a game too would be sensible.