What is a "reasonable" wedding gift, for your own child? Just that really. Adult child, that doesn't live with you or with their partner, currently. (Not asking for judgement on that).
How would you wotk out what's "reasonable"? Based on % of your own income? Based on your own finances? Based on theirs? Based on "fairness" between siblings? Or just a simple flat rate .... in which case "how much"? Give cash? Or a personal gift?