I buy a whole box ($250) at a time. I have done this four times and have only found 6 coins. 2 per $250 of dimes. I don't think I'll be doing it again. I search a lot of halves and have had pretty good success. I find about 1%. I just searched $2000 in halves and found 38 silvers. Never tried quaters though.
I was with sniperdude when he was doing dimes. His first box had a english five pence. Then the next day he bought two more boxes and got 5 silvers and two canadians. I haven't done dimes before. I wanted to today, but I didn't get to it in time...
I was doing dimes exclusively prior to shifting focus to halves...Takes a lot of dimes to make one 90% half! I did find some very nice Mercs though. I would usually get two boxes ($500 total) and get 4-6 silver. Might have to go back to dimes occasionally as halves are getting harder to find and haven't had much luck with ones that i've ordered.