Imagine that bamboos flowered every year. You could grow loads of them from seeds. As it is, you can split a plant every 1 - 5 years (sometimes more!) to get a new plant. That takes a fair amount of time and effort, and hence the high price. Bear in mind that plantings from established groves require a fair amount of attention to ensure that they survive, and even so there can be a significant failure rate! Hence the high price for some bamboos.