You would have to update both the grand total and the individual item totals.
I would want to look at your code to show you where to update it. Are you using shipping, tax, discount, or charge rules of any sort? do those get discounted 30% as well? What you add depends on the very pariticular details of what you want and how your code looks currently.
I would need a detailed description of exactly what you would want, maybe with some samples and I would want to look at the full code block, not just this one line to figure out where it needed to be updated.
Since paypal passes items with the total and validates that the total of the items matches the grand total, you can't just change one number and have it work. You have to be sure your math all adds up at the end for any place values are passed to paypal.