Flexispot E7 Pro is the value king of standing desks. Dual motor, 355 lb capacity, 22 to 48 inch height range, and the frame alone runs about $400–$500 depending on the sale. You add your own top.
For the top I use a 30x60 solid oak from a local cabinet shop ($150) or a laminate top from Flexispot ($80). Either way, you're at $500–$650 for a desk that would cost $1,200 from Uplift for a similar spec. The motor is quiet, the memory presets work reliably, and the anti-collision feature catches your coffee cup before the desk crushes it.
Pro tip: run a good cable tray underneath (I like the CableBox J-channel) and use a grommet-mount monitor arm. A clean desk is a focused desk.