How can I you calculate ROI for one of my products?

Yes, you can calculate ROI of one single product. Take the sales you've experienced because of that product line, subtract all marketing costs and divide by the investment. You can do this manually, but using a spreadsheet might be much easier. Check out this article if you're unsure about the calculation: https://smallbusiness.chron.com/use-excel-calculate-roi-product-44800.html.