The quotes you posted directly contradict at least part of what you contend. "There's a separate (smaller) penalty for total development in a province" in that context means exactly that increasing the base tax of a 10 base tax province is more expensive than increasing the base tax of a 2 base tax province. Remember, in the new system what we're used to calling "base tax" is now "total development".
Also, since there's a penalty for terrain, and very low base tax provinces tend to be hard-to-develop terrains like desert, it's even less likely that you'll be able to have a 20 province nation consisting of entirely minimum-cost provinces.
So if we assume based on the quotes you provided that instead of (5 + 5 * manual development) the cost is (2 * total development + 3 * manual development + terrain penalty), the cost of improving a "1 base tax" province (that is, 1 + 1 + 1 development to start with) by 1 base tax worth of development would be (6 + tp) + (11 + tp) + (16 + tp) = (33 + 3 * tp) total points. Assuming that the terrain penalty across your 20 * 1 base tax province nation is an average of 5 due to having a lot of deserts, mountains, etc, that gives a cost of 960 total points (at least 220 of which is admin). And if we assume flat 4 base tax, it goes up to (51 + 3 * tp) total points for each province, which is 1020 total points even if none of the provinces has a terrain penalty, and 1320 if it's an average penalty of 5. That puts it even farther from your "mere 100 points" than under the previous assumptions.