RX 550 isn't cheaper in the US. Check out this sorted/filtered list on newegg: https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20601296396%20601296397%208000&IsNodeId=1&bop=And&order=PRICE&page=1

If you can get an RX 560 for the same price, or even less, than an RX 550 why wouldn't you? Unless, of course, you could get an RX 570 for MSRP... but that hasn't been possible for a couple of months now.