You have a couple options depending on how high tech you want to go...
Absolute cheapest option with most manual work: Temperature:Get 2 or 3 cheap thermometers and average them. Ph, Alk, Nitrate, Phosphate: Test kits. I always used Red Sea with my previous tank, which is fairly cheap and accurate, and come in a pack. There are pH strips for freshwater, I wouldn't use 'em on marine but YMMV.
Slightly more expensive: Temperature:Same as above, or you can grab a lab-grade thermometer for like $50 if you really want to. Ph: pH probe pen type. They're like $40 now. Alk: Hanna checker Nitrate: Nyos test kit Phosphate: Hanna checker