How to figure out / calculate the best deal?

I get 30% of all sales in free product, and I currently have $229.65 free. I also get 6 half-priced items. I get to include 30% of what I spend on the half-priced items in my free product total, as well. I have a list of 15 items ranging in price from $17 to $62 dollars, with a full price total of $582. How do I select a combination of free & half-priced items to spend the least amount of money? I tried putting a spreadsheet together, but my brain just isn't working that logically right now. =) Thank you!