Irregular adjectives and adverbs

I'm confused about adjectives and adverbs like expensive, dear, costly, dearly, etc. Can you please explain them to me?

Expensive / dear / costly

These adjectives are all synonyms though they are used in slightly different ways and in different collocations. It is also the case that dear as an adjective has two meanings, it means both expensive and well-liked, as well as featuring in expressions such as Oh dear! or in letters as in Dear Sir. The problem with costly may be that it looks like an adverb as it ends in -ly. This is confusing as most adverbs end in -ly, but costly is an exception and is an adjective. Compare the following uses and collocations in these examples:

• It was an expensive suit, but if you want to work for this firm, you have to dress well.
• These are very nice. ~ They're a bit too dear / expensive, I'm afraid. Haven't you got anything cheaper?
• Agatha is a dear friend of mine. She is so kind and gentle in everything she does.
• Oh dear! I've forgotten to bring my ID and I shan't be allowed to take the IELTS test.
• It was a costly mistake and it meant I wouldn't have another chance until the autumn.