Different meanings for phrase "off the regular price"
Solution 1:
30% off regular price = An item for £100 now costs £70
30% of regular price = An item for £100 now costs £30
You could avoid typesetting errors, as mentioned by GEdgar, by omitting 'regular price'. Any native speaker will understand what '30% off' means.
'30% of' doesn't mean anything on its own.