How much is 三百文钱 today?
This should be a question for history not Chinese language.


For the language part: 
In ancient China, multiple kinds of currency are used like gold, silver, copper coin and even cloth. 文 is the unit for copper coin like this Ming dynasty coinage
For the history part:
During Ming dynasty, 1000 文 ≈ 1 两 silver which can buy 2 石 rice = 188.8 kilogram rice = 416.2 pound rice. So 300 文 is about 124.9 pound rice.