An SEO gold rush has recently become the new trend. It's called "domains scraping" and it's about people hoping to find good domains that are free for registration to any demander. They believe that the rivers of the internet are filled with SEO gold waiting to be found. Except that there isn't any gold in those rivers, only a rush. Finding a good expired domain is like finding a straw in a haystack.
There aren't any good expired domains waiting to be found, and the reason it very simple. If a domain were any good, someone (usually big players/companies equipped with experts) would have surely registered it much earlier.
An abandoned domain available for everyone to re-register without any extra fee should raise many red flags, including:
So yes, expired domains are very cheap. You can register them with a typical 8$ fee and become their new legal owner, but many others could have done that a long time before you and chose to avoid this dubious pleasure, probably for a good reason. Using an expired domain is likely to yield zero or even negative SEO impact. We at T.B. Solutions have stopped scanning for expired domains 6 years ago (as for 2015), once we realized they aren't any good.
There are decent alternatives to scraped domains. If you are an expert in analyzing backlinks, you can find good domains in public auctions (e.g. GoDaddy's), but auction are also far from being free of spam, misuse history and other ills. Unless you are an expert in that field, you are still at risk to end up with a negative SEO impact.