Value looks so different to every brand, and also really depends on where that brand is in it's journey. Firstly, Decathlon is one of the biggest sports retailers in the world already, so I don't think a sum of money like that is particularly big to them. Also, it could well be $1 over a 5/10 year period, which is actually a lot more in line with what other athletes might be earning per year.
Secondly, I see this move as a way to level up the brands reputation and standing at the elite end of the sport, than the value they'll get from performances or sales. For a while now they've been bringing in some top-class nationally-ranked runners from across Europe. Good now and potentially future stars, but hardly well-known or groundbreaking signings for the brand. With Cheserek, Chelimo, and now Gressier they're bringing in that next tier of athlete, which makes people take them more seriously as a brand wanting representation on bigger stages.
I expect them to keep building over the next 5 years, and this is a signing made more to pave the way for the future. We may well see them signing the best of the best in a few years time, but right now they're not appealing enough for the super-elites. Think of where Hoka were a decade ago. There's no way they'd have people like Nordas and Grijalva competing for them (and now a British Record in Leonard). It takes time (and investment) to get to that calibre of athlete on board. Similar to On. A few years ago their marquee signing was Joe Klecker, and now they have the likes of Nuguse and Obiri winning Olympic medals. So as you say, value is very hard to define.