How much to give for a wedding is synonymous with your availability for the wedding.
It is well-advised that you split your budget for the wedding gift into 25% / 75%.
25% on engagement gifts and the remaining 75% on gifts for the main wedding.
Something like wine, a box of chocolates or a box of cupcakes will do.
If you will not be available for the main wedding and the couple means a lot to you, follow Diane Gottsman’s principle.
Browse through their engagement gift ideas and pick something that’s equivalent to the amount you would spend on a wedding gift.