About PlugRush CPM

Hello, I use plugrush direct url to sell my traffic, but I dont understand how CPM price is calculated. If I use php header function to redirect my traffic to direct url then CPM price is rougly low 0.05 $/1000 impression, but if I use javascript redirect window.location.href then CPM price is significantly higher 0.35 $/1000 . Problem is if i use javascript redirect then less visitors are sold, because many browsers block popups. Maybe anyone know why prices are so different on these ways? and how can I solve it?