This is an example of a word that really should have been taught in a sentence. Teaching "this costs a pound" and "this weighs pound" would have both taught two useful phrases and avoided the confusion that app users can't escape by checking the comments...
Glad I have learned to check comments because I would have guessed that was an irregular plural form or some such.
I wrote "a pound" because I've learned that in the sentence "They cost 80 pence a pound" you would say ".... y pwys" - but it was marked wrong. Have I missed something or is this just DL inflexibility again?