Ikea Knoxhult - Distance between countertop and tile backsplash

For how many euros can you get a 1.80 m overdeep countertop in the hardware store?

That was not so expensive back then (already 5, 6 years ago). Wooden strips together under 20 euros, treatment under 10, the strips each around 20 (were 2.50 or 3 meters long, don’t remember anymore – needed were 2.30). Then also glue, which was available back then. It did require some time and skill, but a worktop with cutout for sink and hob would have been significantly more expensive (and might still have needed a wall finishing strip).

How many euros do you pay for a 1.80 m deep worktop at the DIY store?

That cannot be generalized. DIY stores are happy to get rid of leftover pieces. So negotiate on site! That definitely works. The price depends on the type of worktop. A solid wood board is more expensive than a cheap board. So go there, have a look or ask.