IMHO....You don't spin because it's cheaper, you spin because you enjoy it....kind of like how you don't knit socks because it's cheaper. It's about the process, and/or the results...and maybe perhaps you could stumble upon roving cheaper than yarn, but remember that you still have to put in a lot of spinning time!
All you need to get started is fiber and a drop spindle. You can make a CD spindle (I think there's a link in the spinning resources thread about that) or buy one -- personally I recommend buying a good basic spindle because the results are more predictable and you're less likely to get frustrated and quit. My all-purpose spindle is a Schacht Hi-lo spindle -- it only cost $16 and it's far superior to the $12 one I got on eBay. It's so versatile and spins so well.
As far as how much roving you need....look at any ball of yarn and you'll see how many ounces/grams it is...so how much roving depends on the weight of yarn you want to spin. So say you start with 50g of roving...a 50g hank can be anywhere from 50 yards (bulky) to hundreds of yards (laceweight)...it just depends on how you spin it!