In principle, any wacom pens that work on the Samsung Galaxy Note line should work. So if you want something really cheap, you could get a pen for, say the Note 3 or 4, which one can often find for less than 10€. Of course, they might not be so comfortable to hold. But then you can also check for other pens that support the Galaxy Notes.
Of course, I can't vouch that every single one will work, but I just tried my Note 9's pen on my Onyx Boox Note non-pro yesterday, which worked just fine. I'd assume between the original OB Note and the current ones they use the same digitizer (possibly not fit the older N96 line). I have two other older pens that work on my Samsung Note device, if anyone's interested, I could test those (one older Thinkpad pen for the Thinkpad Tablet 2 which I actually quite like, but there it can be tricky getting the right version, since all newer Thinkpads use a different, active pen technology).
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