Exactly. TDS pen, $40. pH pen, $100. Knowing exactly what your PPM and pH is immediatly and accurately; Priceless.

Honestly if your doing something for the first time and dont care too much, Id say the pH pen is not 100% neccessary. The big problem is, that nute solutions are not clear so you have to gauge with the readings. For the plain sake of simplicity and ease, all you gotta do is drop the pH pen in for a few seconds and you know what it is. Will the plants grow without having a perfect 5.8 or whatever pH? In my opinion, often yes they will grow, but they definitely wont produce as much or as good of bud. The TDS pen has been a life saver. With my resovoirs, I dont always know the exact amount of water I have since I just use the bathrub to fill. So I go approx with measuring the nutes, and then I can just add till I get the ppm I want.

Hydro is more expensive and more involved, even though you can theoretically leave it for periods of time. You need the proper investment like everyone has said...if you want to help make sure you get a good product.