Spinning your own yarn is addictive. You will have lots of roving, fiber, raw fleece and yarn, and still want to buy more... just like you do with yarn. Trust me. :D
If you're buying ready to spin roving, it's not necessarially cheaper than buying yarn, but not really more expensive either. However, if you want to wash, card, spin and dye your own yarn from raw fleece (read: picking out sheep poo, grass, hay, dealing with the smell, washing the grease out of it and spending hours flicking and carding it) it is generally cheaper than buying yarn... way cheaper for the quality of yarn you can end up with.
But it's a lot of work. I spin just to spin, not to save money, and certainly not time.
Oh, and never say no to a free spinning wheel. A good wheel is EXPENSIVE. If that one is decent, and you can get it fixed for even up to $200, it's a steal
. And even if it can't be fixed, you'll have a wonderful display piece for a knitter's home.