how can i tell when my struts are completely worn out. how much will it cost to replace them

well, bounce once on each corner and if it takes more than like one or two rocks to come back to level, then your shocks aren't so great. struts are cheap, i paid like 180 for 4 kyb gr-2s locally and put them on myself. don't know what a shop would charge but its not hard at all. hope this helps.