Please let me know if there's anything wrong with this spreadsheet, as it's the first one I've done in Google Sheets rather than Excel. The link should prompt you to make a copy of the spreadsheet. You can type the amount of each item that you plan to buy into the column titled "Times Bought," then it will tell you the total cost for that item, as well as deduct that amount of star ore from the total amount at the bottom (which is assuming you earn all possible ore this month).
https://docs.google.com/spreadsheets/u/2/d/1mkMd0pvGI0vQesEw59JTD7p6NDQaKoxdW49f9Gzw8WI/copyHope this helps in your spending!