I think your meant to use after 10am but I'm not sure if you don't ever get a pos if there is any point in using them even when not about to ovulate you should get a line
Regarding cost I used to get a pack of 50 Internet cheapies for like $10 and eventually got it down to a fine art and only had to use 3 a month
It gives you a couple days warning whereas temping tells you after but once you get into the swing of it I recon temping is enough that said I found ttc.much more relaxing without temping or opks once I learnt my cycle and what time to dtd