You can go quite far in Japanese without having to spend a single cent. Wanikani is only a convenience product, because you can technically do the same with an ankideck, you just won’t get the same user experience and the premade mnemonics, which might or might not matter for you.
RTK 1 gives you a sorta questionable base to work off of, since it has no readings or associated words, so you might remember abstract names, but not anything that you would actually use. I’d personally suggest not really worrying about the kanji themselves, I’d instead try to go for a word deck, like the famous 2k/6k one. Of course it’s a lot of mindless SRS, but at least it’s teaching you something you can immediately put into use.
Now, for other stuff, you probably want to read something. Tadoku has a big collection of free books you can try. They are children’s books, they aren’t the most engaging if you are an adult, but they are at least approachable and free. Depending on how much you want to stretch the definition of “single time purchase”, buying manga and potentially even reading along with the ABBC book club would be a good idea.
Listening is probably where the difficulties begin. Let’s assume you want to do things legally. Nihongo con teppei is a pretty common recommendation for beginners, but it can be a bit too difficult if you are just starting out. SImilarly I have a couple post on the forums about interesting youtube channels (Here, here and here), but they are also not the most beginner friendly. You can of course always sail the seven seas to get some listening in, but I won’t be listing any sources for that.
Now, once you get out of the beginner levels and into the intermediate ones, that’s when you would most benefit from just consuming a ton of content. That’s probably where it really becomes difficult to find free stuff, but that’s a bit away, and you might be in a better financial situation by then.