I know Apogee iJam can do this, but it's a little expensive for me. iRig can connect a guitar to iOS devices, but will iRig connect a guitar to a MacBook Pro?
If your Macbook Pro has the single 3.5mm jack that can take input and output then iRig should work just fine for you. Someone asked a similar question here. If you're looking for something ultra cheap you could always get a 1/4" to 3.5mm adaptor to plug your guitar into your Mac. I did this for a very long time until I could afford higher quality hardware. These things run as low as $2 at some places, here is one for example. Hope this helps!