Complex sentence: Unfortunately the mine did not turn a profit, despite his efforts to make it, pouring in more and more money until he had to put the mine to sale; however, there were no takers and he was reduced to near-poverty.
Split into simpler sentences: Unfortunately the mine did not turn a profit, and he poured in more and more money until he had to put the mine up for sale. There were no takers and he was reduced to near-poverty.