I've tried different ovulation prediction tests. When selecting an opk, decide when you want to use it. Some brands are supposedly sensitive enough to detect using fmu, but others should be used midday.
The Answer 20 are the cheapest I found at a drug store. It has 20 test strips for about $15. Reading the tests can be a bit tricky because you have to compare two lines to see which one is darker. Sometimes you feel like you need to make a judgment call.
I stopped using the Answer 20 because I couldn't get a midday positive. In fact, I couldn't even get any kind of a second line midday. I could only get a positive with fmu but it wasn't sensitive enough to detect the LH surge on the first day. As a result, I was detecting the surge a day late and my LH surge was often late relative to my temp increase.
Next I tried the most expensive opk - the Clear Blue Easy Digital (CBED). It gives you a happy face if it detects the LH surge and it is supposedly so sensitive that it will detect a surge even in fmu. The first month it worked great. It did seem to detect the surge perfectly in line with my temp increase. In other words, I had a temp increase two days after my LH surge. The next two months weren't so good. I did not get a single positive in either of those months, whether I used fmu, midday pee, or evening pee. I did however, have a temp increase and regular PMS in those months suggesting that I did indeed ovulate.
Now I use the store brand. - CVS brand. I think it is around $20-$25 per month. This is the only one so far that will give me a positive midday. It's still a compare the two lines brand, but it seems more to work for me. If you start the tests at the right time, you won't need all seven tests and you can spread monthly supplies beyond a month.