For v4.6 I have upped the tax income from civilian mining from 50 to 125 wealth per CMC. It still costs 250 wealth to hire one (actually 375 to hire minus 125 tax but 250 and no tax is simpler). This brings the tax vs profit ratio in line with that from civilian shipping. (assuming 1/3 of corporate profit is taxed). This also makes CMCs attractive as a source of wealth as an alternative to a source of minerals.
Steve