Palgrave Macmillan are offering £30 off on ANY print or eBook in their range until December 31st! This includes titles which cost under £30, of which I found plenty. As far as I can see, there's no way to sort by price, but I just ctrl+f searched for £2 and £3 so it would show me any books that were £20-30ish on each page.
Successfully placed an order, didn't have to enter any payment details and have received a confirmation email, so fingers crossed!
Edit: This website offers a bookmarklet to filter out books over £30 (and also apparently how to get more than one free book, though I haven't tried that myself) - https://hackmillan.github.io/
EDIT 2: The original code is apparently no longer working, but HLDY16S is! :)