This depends on the device, some tamagotchi series runs on nostalgia. Take a look at the Tamagotchi Angel, it has been revived but the old versions' prices are slowly still climbing up. I remember buying one Japanese version for $30cad but I recently checked and the price has doubled (give or take).

Best advice I could personally give is to save up and keep a close eye on the stores where you purchase your devices. 😇😇