Instead of a battery + USB power supply, you have a “normal” power supply. The controller parts of the Deck are basically in the Steam Controller, plus the dock/dongle. The Machine has a huge chunk of metal on the chips to dissipate the additional heat, which also adds to the cost. All of this still needs to fit in an incredibly small form factor and has better, more expensive components plus more RAM in total. Add ~20% VAT because that’s how prices are calculated in the EU.
I say the cheapest we might see for a variant with included contoller is 900€ and that’s probably if we’re lucky.