The Amscope SE120 is the cheapest one I could find for $60 with 20x zoom but it’s a a kids version. The 306 is more advanced but costs $160 already. For one with continuous zoom you’ll need to spend more, swift has some cheap ones. I bought one from Motic which is was quite expensive but I’m very happy, they also offer cheaper models. It’s always worth checking the market for used premium brands like Zeiss, Nikon, Leica or Olympus because those are usually better than China imports (Amscope, Bresser etc) but it depends on your luck of course. I’d see if I can find a used one by reputable brands, buying a new expensive one was good but I could have saved a lot.

I just noticed that I’m quite high, might change stuff tomorrow