Is there a built in feature in OSX that will let me use the 3G signal from the iPhone to let me go online with my MBP? Or do I need a 3rd party program?
will the amount of bandwidth spent be the same as if I were accessing the net with the iPhone itself?
Eddyisgreat said:five gig monthly cap
You can tether your iPhone to your computer provided your provider lets you. It's only in the US of A that this can't be done while you're on AT&T (legitimately, that is). Everywhere else, like here in Australia, I can tether it over the USB cable, or over bluetooth.
Data is data. You will have different browsing habits on a computer than on a laptop which has a lot more freely available broadband at home. So if you're tethering with limited data, then I'd be frugal as needed and consult your 'Usage' on your iPhone via 'Settings.app -> General -> Usage, scroll down'. That is cellular data only, not any WiFi data you've used up.
Oh you can do this legitimately in the US; you just have to pay $20 on top of your existing data plan without getting more data out of it.