The blower motor itself costs around $200.00 to $400.00 to purchase and then if you install it yourself it will only be the cost of the blower motor and your time to install it.
If you hire a furnace repairman to replace the blower motor then it will be the cost of the blower motor plus around $150.00 in labor and service charges.
Sometimes it's best to just have the whole furnace replaced if your blower motor goes out because it can cost just as much as a new furnace.