A donation for Yizkor is a Mitzvah and will be a great merit to the soul of your dear departed. In fact, we say in the Yizkor prayers, that we will "give charity to perpetuate the memory of their souls".
There is a beautiful tradition to donate £18 per name, as the Hebrew word "Chai" - "Life" has the numerical value of 18. Alternitively, any donation, smaller or larger would be much apprecaited.