I used to make custom snap-on covers for 12 years. Mine is currently 9 years old and in great condition. Prices will obviously vary. Material is a big factor.
I recommend spraying silicone on the snaps once a year. Do not spray it on thick as it can cause the snaps to come undone when traveling down the road. The silicone spray helps in preventing the snaps from breaking off.
I use bees or paraffin wax on the cup side of the snaps.