Building can cost less in some circumstances, particularly if you already have suitable tools and skills, but it is not automatically cheaper. Include materials, fixings, scratching surfaces, cushions, finishes, tool purchases and the value of your time. Allow for mistakes and replacement parts as well. Compare that total with a bought product that actually meets the same size and construction requirements. A low material bill is not a saving if the finished tree is unstable or uncomfortable.
Buying may also provide assembly instructions, a defined specification and access to compatible replacement components, although these vary by product. Make the comparison before purchasing supplies. If your budget is limited, prioritising one well-sized resting surface and a stable scratching option may provide better value than attempting an ambitious multi-level tower that is expensive to finish safely. Use the same specification in both columns: a homemade single perch is not a like-for-like price comparison with a complete multi-level tower.
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