Before I get started, I will go ahead and say that spending 70 bucks on a deck is not the worst thing in the world. Depending on how far you go with this, you'll find yourself spending much more than 70 bucks. Greninja is also not a bad pick. It's literally one of the best budget decks that doesn't get you into Tapu Lele GX and can pick a fight with every other meta deck (except grass, but all decks have a weakness so this is unavoidable), but as you mentioned before, it suffers from chronic inconsistency which can be disheartening. I always suggest Greninja BREAK to new players trying to get into competitive because once you've experienced 3 stages of evolution, playing a stage 1 or stage 2 deck suddenly becomes much easier.
There are a few things you should know about the TCG overall. I'm assuming that you're kind of new to this, but if I'm preaching to the choir here, just ignore what I'm about to say.
1. There are a lot of universal cards that almost all decks use. Sycamore, N, Brigette, Ultra Ball, Tapu Lele GX (if you have the money for it), Rare Candy, Field Blower, Choice Band, Float Stone, Double Colorless Energy, etc etc. Once you build your first 60 card deck, you'll realize that you'll need less & less cards to change decks, and ultimately you'll just end up spending money only on new Pokemon. This brings me to my next point.
2. If you are uncertain about the future, the best thing you can do is get yourself a full playset of all of the relevant item and trainer cards and grab yourself 2 or 3 Elite Trainer boxes to get all the energy cards you'll ever need on top of the dice and condition markers, if you don't have them already. If you own 4 Rescue Stretcher, 4 Ultra Ball, 4 Sycamore, 4 N, etc etc, those are one time purchases that you will find useful in a ton of decks. The reason why I like Pokemon TCG so much is that deck building is efficient. Roughly half of your deck will be the same between any deck you play, and that saves a lot of time and money. So now it really comes down to the Pokemon.
3. Research a lot. If you're looking to reduce expenses on this game, you're going to want a lot of knowledge. In fact, you can be competitive through knowledge alone. Look up all of the decks that placed in top 16 at big events like Internationals and Worlds, and don't be afraid to look up other Pokemon that people don't seem to play as well. Then go on youtube and look up decklists or online battles to see if you can find a playstyle that you like.
4. Use proxies or play out decks in your head. Once you've played a large handful of games and seen a handful of decks, you'll be able to mentally play out the first 2-3 turns of any deck list and ask yourself if that's how you want to be doing things. For everything else, there's proxies. Whether that's black & white print outs of cards you tuck into sleeves or just sheets of paper that you handwrite a card's text onto and shove that into a sleeve, play with proxies before you buy for keeps, unless you just really want some cards for your collection anyway.
5. The game is always changing and will continue to change. You will likely own enough cards to play 2,3,4,5,6 different decks after a few months. There is no such thing as a bad investment unless you buy a card that's going to be rotating out of standard in a month and you have no plans on playing expanded. Since a rotation just happened, there's no semi competitive/competitive deck that's a bad investment. On the contrary, you'll never know when an older deck or Pokemon will come back and become relevant.
6. No matter what deck you play, you will gain some experience and knowledge that you otherwise would not have. It's a lot easier to think several turns ahead when you've already experienced a lot of different match ups.
From here on out, you can either look to modify your Greninja list until you find your perfect fit if you really enjoy the deck, or do some research and ask questions.
