Personally, I'd avoid free if you can. Unless of course by free, you mean included in a package such as logic, in which case I'd say the sample instruments in logic are incredible value for money, and more than adequate for doing a song production.
However, it depends how far you want to take it. When I first started working on my instrumental album, the publisher I was working with instantly picked up on the low quality of my samples compared to their more established composers. Long story short, I spent about £350 on samples, and £400 on a new, more powerful computer so I could use them.
You get what you pay for on the whole. But if you pay for logic pro, I think you've got more than your money's worth and then some.