In addition to my Ninja blender, I also have a blender with my Bosch mixer. Due to us making smoothies so frequently, I thought it prudent to get another one that had a bit more power. I found the Ninja 1000w blender with a 72 oz capacity at Target on sale for $85. Kohls also carries it if you shop there.
I did make smoothies my Bosch and it worked well, but I was afraid that the constant, high-power use would burn it out.
The 'rule' or recommendation I heard was that if it is a blender that can crush ice, then you can use it for smoothies.
I wasn't ready to spend or commit to purchasing an expensive blender just yet. Yeah, $85 isn't cheap, but it is less than the $400 for the VitaMix or Blendtec that come in at around 1300w.
So if your $20 blender can crush ice then go for it.