1 MP costs 15 BC to buy, so "producing" only 1 BC instead of 1 MP means taxes are indeed very low.
The problem with this reasoning is that the way that the wealth slider behaves is exactly what you would expect of a spending slider or a tax slider. Increasing the wealth setting instantly increases governmental net income from that particular world, which indicates either spending cuts or tax increases. If it's supposed to represent the government cutting taxes or spending more money on economic stimulus, then the expected result would be an immediate decline in governmental revenue, potentially followed by an increase in revenue later on due to economic growth. Increasing the wealth setting could arguably represent tax cuts and spending cuts, though the net effect is still that the government is taking more money out of the local economy than it's putting into it, which I still don't see as something that should necessarily make the populace more happy - after all, given that the military upkeep is not reflected in planetary income, planetary revenue increases from tax cuts and spending cuts indicates that the government has cut local services, such as health, education, sanitation, or law enforcement, or spending on local infrastructure, such as highways or spaceports.
Beyond that, the 15 billion credits per manufacturing point is an incredible increase in the cost of production when compared to Galactic Civilizations II, where each point of manufacturing cost 1 billion credits out of a factory. It's not an incredible increase in the cost of contracting the work out, however. Given this information and knowing that a world with equal wealth and manufacturing multipliers will trade 1 manufacturing point per turn for 1 billion credits per turn, do you think I'm going to assume your figure of 15 billion credits per manufacturing point or something closer to the GCII figure of 1 billion credits per manufacturing point?
Of course, the new system also messes this conversion up a bit because there exist worlds where the multipliers differ, in which case I could be trading several manufacturing points per turn for 1 billion credits per turn, or several billion credits per turn for 1 manufacturing point per turn. Even so, I still see no reason to assume that the cost of a single manufacturing point is all the way up at 15 billion credits, nor do I see why the wealth setting, which happens to be the setting you turn up if you want to squeeze more money out of a planet, should at all translate to a lower tax rate.