Since Draft2Digital only prints in grayscale, the only thing that is going to affect your print cost is the number of pages. This means the only way to reduce the print cost there is to reduce the number of pages, whether by using a larger trim size, reducing the margins (caution would have to be used here to ensure you're meeting their minimum margin requirements), reducing line spacing, orotherwise adjusting the content so there are fewer pages.
You can find Draft2Digital's print calculator here: https://www.draft2digital.com/podcalc which will allow you to see how adjusting the page count might affect your profits.