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100

Once in a blue moon

Doesn't happen very often

100

Hit the hay

Go to bed

100

Costs an arm and a leg

Way too expensive 

100

Kill two birds with one stone

Accomplish two things at once

100

Don't cry over spilled milk

Don't make an emotional outburst over something small

200

Make a long story short

Come to the point, leave out the details

200

Every cloud has a silver lining

Every bad thing has its end; bad things have something positive about it

200

The best thing since sliced bread

Something outstanding; really good

200

Cut corners

Making something easier by avoiding or skipping steps

200

Blessing in disguise

Something looks bad at first, but has a positive outcome

300

Bite off more than you can chew

Taking on task(s) that are/is WAY too big or complicated

300

At the drop of a hat

Do something instantly 

300

Back to the drawing board

Almost complete, but you have to start all over again

300

Don't judge a book by its cover

Don't judge someone or something just by its looks

300

Feeling under the weather

Not feeling well; sick

400

Give the benefit of the doubt

I'm going to let it slide without any proof

400

Barking up the wrong tree

Blamed the wrong person

400

Cross that bridge when you come to it

Don't worry about it until it approaches/happens

400

The last straw

The final time to allow the worst to happen; last time for something to go wrong

400

Taste of your own medicine

Doing something bad back at someone who wronged you

500

Don't quit your day job

You're not good enough to make money doing something

500

Don't put all your eggs in one basket

Don't put all your resources on one possibility 

500

Don't count your chickens before they have hatched

Don't plan things that will not happen/come to reality 

500

A penny for your thoughts

Want to know what's on your mind

500

Hit the nail on the head

You're exactly right