why increase in working capital = cost?

In calculating present value of net cash flows, I guess you start from the net profit figure from the profit and loss accounts, and then you need to know whether change in balance sheet items means cash inflow or outflow, and then add to or minus from the profit to arrive the final cash flow figure.

Increase in working capital has many possibilities, may be increase in debtors, decrease in accounts payables and so on. Increase in debtors means that you sell goods but has not yet received any money, decrease in accounts payables means that you pay money to the creditors.

Thus, increase in working capital means that you need to deduct the increase from the net profit figure to arriave at the cash flow. This means ncrease in working capital represent cash outflow, not cost as stated in your question; there may be a misunderstading of cash outflow as cost.

Just a guess from your question