The best inexpensive tree is one that meets your cat's essential needs without charging for features it will not use. Start with usable sleeping space, stability, suitable load limits and an accessible layout. A cheaper model with small beds may be poor value for a large cat even if its overall height looks impressive. Compare the complete delivered cost and check whether assembly, replacement parts or washable covers matter to you. Customer photographs can show proportions, but they cannot prove structural performance or suitability for your own cat.
Avoid buying purely because a discount percentage is large. If your budget cannot cover an appropriate tower, a simpler safe arrangement may be preferable while you save. Price is one comparison point; a product that your cat can comfortably use for its intended purpose is the more meaningful definition of value. Set a maximum total spend before shortlisting and remove models that fail an essential requirement, even when they are cheaper than the remaining options.
For product comparisons alongside this advice, see Ultimate PawHouse and Taj-MeowHall. The details below explain their sizes and uses.
Further reading: cats.org.uk; rspca.org.uk.
Ultimate PawHouse
Taj-MeowHall
Cat-Cave