As cost/time/maintenance matters so having both at a single place is a good choice. Sometimes you may get a hosting deal since you bundled (for maybe for a certain period) or at minimal charge along with the registration.

Hosting cost is normally a monthly expense and it can range from $10 to $35 per month. for basic hosting At times a company may sell the hosting service for less than at some introductury or restricted rate.

Domain registrations can normally cost $20 to $35 per year, there can be a discount rate for a special domain type.

If your domain and hosting service are seperate, you can join them under one company allowing you to possibly recieve a discount for two services provided.