So sorry for your loss, Marydoll.
Now I feel guilty, because I never offered anything, or suggested doing so, when I rang my husband's former confessor and asked him to make an Intention for my husband's soul, shortly after he died.
I second those who say, whatever you give will be accepted willingly, so I realise this doesn't help you. Perhaps if you can afford it, you should go for somewhere in the middle of the range you have seen offered in your time as a sacristan.
I sent a larger donation to Caritas just recently for use in Ukraine, as I know my husband would have wanted that. So another guideline could be what the friend or relative would have considered suitable, if you know what that is.