Amsterdam - Paris has one of the most expensive seat reservations : 32€ 2nd class or 37€ 1st class (+2€ booking fee directly added).
Please note that Thalys (and Eurostar) have a limited quota for passholders and can sell out weeks in advance. What would be your travel date ? You can't see availability on eurail.com until you've bought a pass. Best check here for Thalys : https://www.b-europe.com/EN/Booking/Pass#TravelWish
There are alternatives such as reservation-free IC to Brussels + Thalys from there (27/32€) or ICs to Lille and TGV to Paris (limited 10€ in both classes, then 20€).
Let us know for questions.
In general : the seat reservation fee is often the same or a bit more expensive in 1st class.
Very expensive : TGV to Italy (45€ 1st class) or Switzerland (70€). Best avoid them and travel on any TGV up to the border point (10-20€ in both classes).
And the other way around (good news !) : seat reservations are free for 1st class passholders through tickets.oebb.at in Germany, Austria, Czechia, Hungary,…
It is advised to avoid eurail.com for booking seat reservations mainly due to extra fees + more expensive optional seat reservations. Have a look throughout the forum and here too :
https://www.seat61.com/interrail-and-eurail-reservations.htm