The cost to have a lawyer on retainer depends on the lawyer and their fees and how much work they may need to do to be on retainer and should they need to help you out if you need the lawyer.
Most costs for a lawyer on retainer fee is between as low as $500.00 to as much as $5,000.00 the retainer fee can be either paid monthly or yearly depending on the lawyer and services you'll need.
Mostly business who need constant legal work will hire a lawyer on retainer so they can have the lawyer available in the event they need them.
For most people though a lawyer on retainer would be a waste of money but for businesses or someone who is gonna need a lawyer for something having a lawyer on retainer is or could be worth the cost.
You'll always be able to call up that lawyer and get the services from them without having to find a lawyer.