So the two (cheap) transport options seem to be to walk either to a Dutch bus stop or to Weeze train station:
Weeze train station
Tickets from the train station to Amsterdam typically seem to cost around 40 euros, but sometimes are available for 25 euros if you book early and are lucky timewise. Tickets can be booked on bahn.de. As far as I can tell there is currently a train replacement bus leaving from the train station, but I believe this temporary.
Dutch bus stop
From the Dutch bus stop (Bushalte Kasteellaan, Well L) you can either take a bus to Venray with Arriva or to Nijmegen with Breng costing 2.83 and 8.12 respectively. Note that without a Dutch OV chipkaart this price might be higher (unless you figure out a way to buy the ticket digitally, Breng works with the Tranzer app and Arriva with the Glimble app, but I didn't check prices in there).
From Venray the normal train price to Amsterdam is 24.89 . If you buy at the train station it's one euro extra, but you can use the NS app instead. If however you buy through Gaiyo and you travel outside rush hour (The off-peak hours are on weekdays from 00:00 - 06:30, between 09:00 - 16:00 and between 18:30 - 24:00 the next morning, plus all day at weekends) it's 10% off for 22.40, for a total of 25.23 euros.
From Nijmegen the normal train price to Amsterdam is 20.70. With the 10% off that works out to a total (including bus) of 26.75.
But: There are often (not right now) better discounts available for Dutch trains which are often fixed price regardless of the distance, which can always be found on treinreiziger.nl (use Google Translate).
Times when the best connections happen can be found on 9292.nl from "Bushalte Kasteellaan, Well L" to "Amsterdam Centraal". Travel time is 2.5 to 3 hours.
Take the German connection if it is available for the 25 euros, especially if it's fully by train. Otherwise check treinreiziger.nl and 9292.nl for the best connection.