What parts of the Pokémon TCG are you most interested in returning to?
What I suggest is a terrible idea if you're all about attending League, tournaments, etc. because it won't allow that. The best budget-friendly way to return to the game is the Pokémon Trading Card Game Online. It isn't perfect, but starting out with it, grinding for rewards using one of the provided Theme Decks, shifting to one of the recent Sun & Moon-era Theme Decks (most of the SM-series decks are among the best Theme Decks) to grind better and get some cards you'll need later on anyway, and eventually working your way up to a "real" deck.
You can do this purely through investing time and not money. If that isn't a priority, then you could also just dump money on buying redemption codes (stick to reputable dealers!) and then trade for what you need (if asked, I can provide good beginner trading advice for the PTCGO). You can also do a combination of both and have it work out just fine.
To return to the physical TCG, you might pick up two copies of the Mach Strike Theme Deck. It is just a start, but skimming out the vital bits will leave you just needing to get some familiar faces like N and Professor Sycamore, plus a few new faces that will become familiar, like Guzma. No, I am not describing a top tier deck, just something pretty easy to use for easing back into the game. If you can find some of the Battle Arena Decks at a decent price, it is also a fairly painless way to get some old staples.
I'll leave the rest to others, especially as we just had a new set drop, but it isn't tournament legal yet. What was working until now might not work as by the 16th (when SM - Ultra Prism becomes tournament legal).