The first step in establishing a budget is tocompile a tentative list of what you think you might need. In some cases, this is a complete, self-contained apparatus, like "Cavendish Apparatus". In other cases, you will need a list of pieces, like "Spectral Lamp, Filters, Photomultiplier tube". This list will definitely change as you research, but start with just the biggest pieces.
Once you have a sense of what you need, determine if we already have the equipment. Be sure -- it's happened in the past that someone found out the week before his paper was due that we didn't have a key piece of his experiment!
If we don't have what you need, you need to find a vendor who sells it and get a price for it. It would be great if you could find the cheapest place, but that's something I can help with once you have at least one price quote.
Here are some companies we commonly turn to for physics equipment.
See also the paper catalogs next to the science fiction books in the physics foyer.