Although it may seam more difficult to close your foundation vent with a concrete block it is actually not that hard and far more cost effective. The franchise companies sell their “vent covers” for around $25 each. To close your vent with a cement block will cost you about $3. Vent covers will not keep out the winter temperatures or keep the mice out (I have see them chew through metal). Cement block are easily accessible at you local home improvement center as well as the mortar or cement caulk and backer rod.