Most local bike shops should have a Shimano replacement bottom bracket for $20 to $30 and might charge $25 to $35 to remove your old one and install your new one. First get the correct dimensions for your existing BB then call a shop to see if they have it in stock and if yes find out the price for the part and cost to install. If your BB is seized then labor cost will be more as more time will be needed to remove the old one. If the threads on your BB shell need to be recut there will be more time and cost. Hopefully you'll find a LBS and get yours replaced for $60 give or take.