Has been slightly cheaper according to the Camelizer, but still a bargain. Far cheaper than elsewhere. Currently hooked on EO4, and have purchased a digital copy of EOU1 from the eShop for the permanently cut-price of £8.99.
Can't see it staying this low forever. Suspect Amazon might be clearing stock--this title won't have seen a huge print run.
EDIT1: Has increased in price to £20.02, so free shipping if you're not a Prime member. Still a good deal.
Absolutely agree with edarkness that if you're new to the series you're better of trying the demo for Etrian Odyssey IV to see if you like this style of game and purchasing the eShop version for £8.99.