I've seen this from both sides - as card-signer & giver of cash; and as recipient of the card and gift. I've also worked in the OP's country and other places in western Europe, and as far as I can tell the expected behaviour is pretty similar everywhere that I have experience of. I have not heard of any companies preventing coworkers collecting money for a small gift for someone who is leaving; nor have I seen any company-mandated protocol on this. In any case, there does not appear to be any formal etiquette in the matter
As card-signer and potential giver of cash, what I have usually done is take a look at the card first and see if I know who they are. If I've no clue who they are, I'll pass the card on unsigned. If I know who they are, I'll write in the card. As for giving cash, I tend to give more to very close friends, but I'm also not embarrassed to give nothing if I don't know them that well. If I'm going to donate something, a good guide I use is to take a peek in the envelope and judge what the average donation is. Some people might have been generous, maybe others gave nothing, but €20 and 20 greetings in the card suggest about €1 is "about right" for a donation for a departing colleague.
As recipient of a card and a gift, my most recent experience was when I left my job in the Netherlands. On that occasion out of ~40 people (who I presume might have had the card passed to them), around 30 signed the card, and the gift had a value of around €30. This, kind of, confirmed in my mind that perhaps I wasn't being particularly stingy by giving €0 - €2 when I was the one doing the donating.