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The book is $2 cheaper at our shop.

The book is $2 costlier at our shop.

Are the above two sentences fine to native speakers?

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Yes, you can certainly write an amount before the word “cheaper” like this.

Other examples:

“it’s 2cm shorter”
“that way is 1km longer”
“my house is 50 years older than yours”

However, there is an error in your second example. We say “$2 more costly” and never use the word “costlier”. (Actually, in ordinary usage in Australia “$2 more expensive” would be far more common. But this may depend on the variety of English.)

This is a complicated and illogical part of English. Learners just have to remember for each word when you use the “more —-er” structure and when you don’t.

Other examples:

10kg fatter - correct
10kg more fatter - incorrect

10% intelligenter - incorrect 
10% more intelligent - correct

For some, both are correct. Such as, “simpler” and “more simple”.

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  • I’m the USA we would say “more expensive” as well. Or simply “$2 more” – Timinycricket Dec 16 '19 at 4:04
  • Yes, I agree: “$2 more” – Orbital Aussie Dec 16 '19 at 5:44
  • In kind of ironic. – Timinycricket Dec 16 '19 at 6:06
  • We would say more expensive in Britain too. Costly usually implies very expensive, such as luxury goods or something that is much more expensive than was expected. – Kate Bunting Dec 16 '19 at 9:01
  • In BrE 'costly' can be a good word, whereas 'expensive' is a bad word. So diamonds are costly ( they cost a lot but are worth it), but buying whisky at my local 24/7 shop is expensive (because they charge more than regular shops for the same product) - but if you are desperate for whisky at 2am you may think that the high price is worthwhile. – JeremyC Dec 16 '19 at 23:00

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