How can you plan long range resources and budgets when using Agile methodology?

Question

Agile does not encourage a lot of up-front design. This is good from a requirements management and software development standpoint, and allows the project to adapt to changing business needs.

However, how does one do any long range planning of resources if you don't really know what you're going to build when you start? Oh sure, you have a conceptual model of what you're going to build, but you don't have any measurable detail from which to gague how many resources you will need to complete the project, or how much it will cost.

Does anyone have any suggestions on how to go about long range planning in an agile environment?