Whats the best fertility test to use, but that is good and cheap?

Your choice. I assume you are talking about ovulation predictor test kits. There are many brands out there and most are reasonable. So you take a pick.
Consider BBT. "best" is relative, but the basal body temperature (bbt) is most economical. All you need is an accurate and precise thermometer and a piece of graph paper. Check your temperature each morning before you even get up and plot it on a simple graph. Your basal temperature will increase on the order of 0.3 degrees after ovulation and stay elevated until your period.