Desalinized water costs between $2.50 and $5 for 1000 gallons (down from closer to $9 a decade ago).

Freshwater costs around $2 for the same amount.

This means your water bill would go up from 25-150% if you switched completely. Even if you don't use that water you'll get charged for the creation of the plants. About $85 of a $150 water bill is for infrastructure and administration (I have to pay this even if I don't use a drop of water). The water I actually use only accounts for less than half my bill.