I believe in general you should always maximize the current cash flow subject mostly to one condition only: whether mining particular ores technically sterilise others. As you've intimated, there is time value of money.
However, it is a fallacy to think that cheap ores should be preserved for hard times and more expensive ores should be mined first while prices allow. Apart from the time value of money and funding that you've mentioned, in the case where prices may later fall below the cost of expensive ores, from the strictly financial point of view it is even more compelling to mine the cheap ores now! You can figure it out yourself with a hypothetical example, it's just simple arithmetic.
Of course, sometimes there are reasons for mining the expensive ores first, but "preserving the cheap ores for hard times" wouldn't be a good one. Maximising the amount of ores mined does not always equal maximising the total cash flow, even if the time value of money is ignored.
(20min delay)