But is there any significant performance improvement in adaptor + m.2 SSD compared to 500-600 Mbps speed of SATA SSDm.2. sata ssds should cost nearly the same, I guess if you find m.2. sata ssd and adapter for less cost, you could save a few bucks. Look for MX500, its a widely available M.2. ssd.
You can go for cheaper ones as well, WD Greens, Kingston etc. if you just need dirt cheap something that gets the job done.