I just negotiated start up with a few places and a general guiding principle I kept hearing from my mentors is to ask for everything you would want (not need, want) that will allow you do your best research. If they are offering you the position, they will not retract their offer because you are asking for start up to do research. The worst the chair/dean can say is no.
Also, keep in mind that start up packages vary a ton depending on the type of school and your area/research focus. R1s, as would be expected, provide much more money for start up because they expect a more productive research program. Neuroscience/vision/psychophysiology folks typically get more start up funds because of the equipment they will need to buy to set their lab up. Even within a content area, your individual research focus will greatly influence start up packages. For example, a clinical psychologist who uses neural correlates of psychopathology using fMRI will need much more money than a clinician who studies treatment outcomes using behavioral or longitudinal designs.
Also also (return of the also), keep in mind that start up funds are meant to be used to start a lab. It is not meant to fund your research program for 10 years. After a period of 3-5 years (this seems to depend on the school), you would be expected to receive either internal or external funding for your research program. Thus, detailing your 10 year plan might not be the best idea because you really cannot be sure what your research program will look like then (what if a series of studies do not work? or you find an unexpected result somewhere along the line?). It's probably a better bet to keep within a 5 year plan.
With all that being said, if you're asked about start up in a phone interview they probably just want to get an idea of the range that you're looking for. This issue usually does not come up in earnest until an offer has been made.