Buy a coffee table from Walmart. As I’m writing this, it’s $19 online. Use this on top of your regular desk.
If you want to switch easily between sit/stand, buy a monitor mount and clamp it to the edge of the coffee table mentioned above. Depending on your monitor, the arm can cost anywhere from $25 to $100. You can then raise or lower the mount whenever you need to sit or stand.
You can buy an IKEA coffee table for considerably less, but it doesn’t have the same edges you can clamp a monitor arm on and could potentially tip over.
Enjoy your cost effective alternative! It works really well for me and I’m 5’9, but results may vary for others. I’ll see if I can post pictures of myself using this later.