I have a question:
Is there any price difference between STEM and humanities right now in the US? Around here price is loosely based on cost (mind you, all education is subsidized so we supposedly pay 30 to 60% of cost) and there's a few categories: studies that involve more practical work, labs, etc (such as the ones in STEM programmes) are a bit more expensive. That's natural to a point. If you don't have anything like it, you are already compensating, aren't you? If there is no direct relation between cost and future salary, you already have an incentive to study jobs with higher salaries.
(Here, the requirements to renew a scholarship are a quite lower -pass 60% of subjects instead of 80%- for for people in engineering programmes. I was always offended that they made the lower requirements for engineers, as if engineering was easier than physics! I am now offended because I see engineering is seen as much more important to society).
Besides all of this, I've come to think that the problem nowadays is in primary education. You have a problem of demotivated or lukewarm university students, and you have that problem everywhere. Really, the medium term solution is to drastically improve their education before they get there. You'll probably be able to get better quality professionals later on, plus if you consider there's not enough interest in STEM you can change that at this level (whereas a purely economic motivation has all the inconvenients already discussed for the field itself)
Ps.: I just woke up, I'm sorry if you need to make an effort to see my actual arguments somewhere in these words.
EDIT: "Some universities do charge more for STEM degrees, because they are typically more expensive to run." I guess I should've read the article first...