How were the sales before volume 57?
Are they in the same range as the sales are now?
Yes, One Piece's sales have been showing a considerable decrease over the past few volumes, since around volume 90 (volume 91 showed some sort of recovery but sales started decreasing again after that). Last 5 volumes before Strong World and last 5 volumes (you can see volume 56 already showed a glimpse of a boost).
|1045937 (4)|1320972|1422906|*********|1540949┃2,169,438 (522)┃2008/12/04|One Piece 52
|1194145 (5)|1476027|1589034|1641952|1682869┃2,279,892 (432)┃2009/03/04|One Piece 53
|1171142 (4)|1450417|1550859|1609125|1650415┃2,223,695 (347)┃2009/06/04|One Piece 54
|1121685 (3)|1487845|1597912|1662688|1696393┃2,345,564 (444)┃2009/09/04|One Piece 55
|1363868 (3)|1762418|1865059|1924398|*********┃2,480,124 (353)┃2009/12/04|One Piece 56
|*231555 (0)|1386360|1604070|1710034|1776868┃2,112,734 (259)┃2019/03/04|One Piece 92
|1378177 (4)|1716100|1855702|1923919|1966556┃2,200,000 (137)┃2019/07/04|One Piece 93
|1182323 (3)|1531766|1711240|1786973|1828286┃1,993,670 (*94)┃2019/10/04|One Piece 94
|*883228 (2)|1452119|1615830|1685705|1728213┃1,955,897 (142)┃2019/12/28|One Piece 95
|1070591 (3)|1433765|1542758|1594700|1633236┃1,688,363 (*45)┃2020/04/03|One Piece 96