Some vintage pedals are very highly demanded and the Deep Impact is one of the most desirable. Like everything else that's for sale these things will find their own value. If an item is scarce and someone wants the 'real thing' then there isn't a substitute.
I really wanted an original Mutron and paid good money for one. To my ears it produces a sound no other filter type pedal can. However, the big pleasure for me is that I love the fact it's old, albeit only in the world of pedals, it looks really cool and it makes me smile every time I use it.
To me that's worth every penny and I think, for some players, the Deep Impact fits into exactly the same bracket - lots of things can do synth sounds but none of them actually is an Akai. They aren't made any more = the price starts to rise.
They are fetching big money even over in the States. Saw one on US EBay got for $850 not long ago.