Bamboo economics are not intuitive. Normally we think of plants that grow only up, but bamboo grows fastest horizontally, rather than vertically. It is the spreading nature of bamboo that makes it so attractive.
Unusual Growth Pattern
Once a bamboo shoot reaches full height and produces leaves, it never gets any taller or larger in diameter. Its growth is complete. So how fast does bamboo grow? It doesn't grow at all! But each year the new shoots are larger and cover a greater area.
Source VERY Important
All container bamboo plants are not equal. The growth rate of a field dug plant is much faster than the same size plant which was nursery propagated. Nursery stock is immature bamboo shoots from immature rhizome. It may take many years until you see mature shoots. Field dug plants are small shoots from mature rhizome. As the rhizome spreads out, full size shoots are possible much sooner. Do not be fooled by lower priced mail order stock!
Size Economy Unexpected
The economics of large vs. small plants are very much leaning toward large plants.
Small plants will never grow larger and they produce small shoots the next years.
Large plants already look good and will produce larger shoots in the following years.
A small plant can be dinner for a gopher in just one day. Gophers can slow the spreading of large plants but not kill them.
Large plants rapidly spread, even in marginal conditions. Small plants die if conditions are harsh.
For the same dollars spent, privacy is created with large plants in about 1/3 the time as small plants.