All I can say is that the only way to prevent pregnancy and transmission of STD's 100% of the time is celibacy and abstinence.
Of course that's not reality for the vast majority of people ... Therefore, if cost is an issue go to your local health clinic ... Many of them provide various methods of birth control for little or no cost.
Regardless of what the girlfriend says she's using, don't ever have sex with any woman without wearing a condom! That way if she's lying, you're covered, and if she's not lying, you're both covered.
Again, nothing's 100%, but one or two methods is better than nothing at all ... Unless you're into abstinence and/or celibacy, that is!
I wish y'all the best, and I think it's awesome that y'all are being very rational about this instead of getting lost in the heat of the moment and having later regrets!