It's taken a long time for the cost of implementing a data warehouse to decrease to a level in which a small business can afford such a project. The signs are clear that this opportunity is just around the bend, or perhaps already here.
One of the signs is that Microsoft is putting downward price pressure on business intelligence tools. These tools were once the domain of the SAS, Cognos and Business Objects of the world. These vendors were of the type that would not publish a price for their product, since each implementation was unique (i.e. the army of consultants would need loads of billable hours to feed the bottom line). With Microsoft releasing SQL Server 2005 and greatly reducing the complexity of designing and maintaining data warehouses, the foundation for business intelligence is in place. With Microsoft releasing PerformancePoint Server, the complexity in monitoring (history), analyzing (the why) and planning (prediction) is greatly reduced.
Another sign is that data warehouse projects have been proven to the corporate world. Financials, logistics and even Wal-Mart have shown the efficiencies that can be gained by utilizing such data. Each of these industries had a need to better understand the "why" of their operations and not just the "what". So, if you ask, who can afford to build a data warehouse? I would respond: Anyone interested in the "why".