Oh my bad. I misunderstood.
Well going the cheapest route, I normally would say get a USB mic for your computer and use something like Audacity, but I have always had some horrible trouble with USB Mics. The next cheapest route, IMO, would be a inexpensive interace under $100 with an inexpensive mic under $100 (or perhaps exactly $100). It sounds like you aren't going for crazy quality right now anyway. You can worry about that down the road as you work on your delivery.
BTW that 2Pac instrumental is exactly what I'm talking about. People have taken away the parts that are in the middle of the song (as in not panned to the left or right) and while that gets the main vocals out, you're missing the kick and snare.