The main thing is to be honest with yourself about whether you're willing to pay not for the toy itself, but for its history and rarity. Because that's essentially what hunting for expensive versions is all about. If you bought a Squishmallow for 200 rubles and you like it - great, you got your enjoyment. But if that same character from a limited series costs three times as much, you're paying for the release number, not the quality of the toy. And that's where things get tricky: prices on the secondary market jump depending on trends, new releases devalue old series, and what was rare yesterday is already nobody's wanted today.
My advice is to collect what actually grabs you, but at the same time keep an eye on the secondary market without getting obsessed. If you come across a rare version at a reasonable price - grab it, but don't hunt for it specifically. Now, if you like a certain character and you see that his earlier series cost more because of rarity, not because he's sold out and they've stopped making him - that's a different story. Then you can take the risk, because interest in the character won't die down.
In practice, the most expensive collections are put together by people who started out picking based on love for the toys, and rarity turned out to be a side effect. Hunting for hunting's sake usually ends up with a shelf full of expensive faceless stuff and an empty wallet.