The RX series is simply a generation, and by itself doesn't designate low end or high end.
The RX 550 is the lowest, and the RX Vega 64 is the top end.
Low to high they are:
RX 550
RX 560
RX 570
RX 580
RX Vega 56
RX Vega 64
However, given your budget, then RX 560 is the highest you can get.