For a domain purchase on wordpress.com it cost $18 per year. But then hosting is free.
And on blogger too, hosting is free and if you need a custom domain you can get it from godaddy at ₹99 for the first year.
In a long run you have to the pay the same everywhere for a domain, its the hosting cost that differs.
A self-hosted one provides the best customization options but then there are little problems in there too. You will need to take care of the security of the blog, also if your traffic increases a lot your current godaddy server might not be able to take the load and you will need to upgrade to a higher hosting plan.
There are pros and cons in every platform. In the end its a personal choice.
I currently blog at wordpress.com