Just do your research, I consider a premium domain name a .com that has a lot of exact searches, and is a good investment.
If you are buying the domain to develop (that's what I do), research the keyword thoroughly, look at the price of the domain and compare it to your estimated ROI on your developed site.
If ROI on a domain is less than six months, I consider it a bargain. For example if that domain is $3,000, and you can make $10,000 per year from it, it makes it a worthwhile investment.
You don't need to pay $3,000 for a domain to make money though, if you dig hard enough you can get aged, keyword rich domains with exact searches for the reg fee.
But you have to have development and monetization skills to make the money in the first place.
Not only are exact match domains good for SEO, Google adsense gives a high quality score to domains that are exact, with pages that are relevant, making the price you pay per click lower, pushing up your profits.