Over or under half price
Solution 1:
We're talking about the price, not the discount.
£100 is less than half of £300, so it's less than half-price (or "under half price" if you prefer).
For a sale price, where a single shop reduces the price to under half, it might advertise "More than 50% off" and emphasise the size of the discount. You're less likely to see that when comparing different sellers.