Build quality and longevity. After five years of daily use you will be able to resell your Brompton for 80% of its initial price, while I would guess you would be lucky to get more than $100 for your old Decathlon.
The most you can lose out on the Decathlon is those €289, whereas 20% of the quoted Brompton price of $3800 is double that :)
Resale value is important in the sense that you know you are risking at most X, which with Brompton sounds to be €600 — in case you dislike it or don't have a need for it anymore, or are in a need of cash, so the risk you are willing to take is still larger.
Ultimately, as with anything, if Bromptons are really of such a high quality, the best purchase is a used Brompton which you might be able to resell for the same price you got it at. If you really end up using it to the max, you can resell it early and get a new one specced to your liking.
For something you'll end up using daily, the most important thing is quality, and then you should look for the best price next.
