Woocommerce product backend page add markup(%) field in which add percentage which you want to increase price of product using plugin. if you want to decrease by some percentage add markup(%) field value as nafgative.
1) Product price $150 and if increase value by 20% in backend add 20 and its display price in frontend $180.
2) Product price $150 and if decrease value by 20% in backend add -20 and its display price in frontend $120.
Wocomeece frontend single product before markup page.
Wocomeece backend single product page.
Wocomeece frontend single product after markup page.