Is it possible to have an affiliate marketing blog that is free, and still earns you money?

Or do I need to pay for a more ‘professional’ blog domain?

Also: Is it worth it to be an affiliate marketer? How much could I earn with a free blog, working part-time on it, and not paying anything to increase my blog search rankings? Is running an affiliate marketing blog a worthwhile thing (financially) to do when I can only do it part-time – or is it something that needs a full time worker?