It will be about half that. You only make 5% when buying at market (middle) price and selling at maximum.
Specifically, if you buy 5000 coal for 196 (market price) for 980,000 and sell it for 206 (maximum price) for 1,030,000, you make 10 x 5,000, or 50,000 on your near-million investment. You actually make about 5.1% here because the 5% limit is rounded up slightly to 206 (it would technically be 205.8 if it were exactly 5%).
If you do it this way, I'd recommend selling the coal for one coin less than maximum, as yours will sell before all of the coal listed at maximum.