What make a Multi-Family a good investment ?

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Hello BP 

Recently I wonder what criteria would make a duplex or Four plex , a good investment ? 

How much is good price for duplex, Four Plex ? how do you come up to that price ? 

What Cap Rate is consider good  ? why you think so ? 

Do you use 1% , 2%, or 50% rule on duplex four plex ? 

I live in Twin cities Minnesota and the price for Multi Family has gone up so much that I wonder if it is still worth investing or at what price point I should stop buying. 

Thanks