You can build your own system for less than $150 from cheap parts on ebay. You can buy a branded unit at a big box or amazon for $150-250. Or a "health" branded version for $300-800. Or have an installer put whatever they sell in for $500-1500.

My experience is that you will get nearly identical water quality from each of those systems. There are different options and some fine details but the fundamentals haven't changed in decades and you are paying for some collection of service, parts quality, future replacement costs, marketing and snake oil.

NSF certification is good, it will rule out products that are flat out harmful. I have seen lots of cheap filters with fake certifications and there are many great filters that it don't carry certification. NSF material and safety cert (51) is a good one to look for, beyond that is has more to do with how the product will be sold and marketed than a real measure of performance.

$250-500 is probably the right price range for a diy install unit. Check for replacement part costs, buy something with standard components and cartridge sizes. Learn how it works, change the filters on time and expect to replace components every now and then.