I heard from a friend that master's programs tend to be more expensive because master's students don't have advisors or as much university funding for their time in academia. However, sources online seem to only indicate to me that the PhD is more expensive. Can someone clear this up?
Masters students are often unfunded. That is, they have to pay (most of) the tuition fee. Against that, PhD students are typically funded. That is, they are paid to study. Here's an example. The student here is not only paid a monthly stipend of $2000/month, (s)he doesn't have to pay tuition fees.
In other words, yes, it's often cheaper to do a PhD than a Masters. Caveats apply: it's possible you may not be offered funding (in which case a PhD is also more expensive). Also, since a PhD takes longer than a Masters, if you graduate with a Masters and then get a well-paid job immediately, the extra income might be enough to compensate for the price difference.