Not sure if policies in academia are relevant to you, but it's $5,000 at Georgia Tech. More specifically, it's $5,000 per year per vendor for supplies/materials/services, and $3,000 per item for equipment (I don't believe the $5,000 yearly limit applies to equipment, so you can e.g. buy five $2000 computers from Dell without permission). From (http://www.admin-fin.gatech.edu/business/purchasing/0500218....):

  The PCard [credit card] may be used for:

    Equipment: Single units under $3,000.

    Supplies, Materials, and Services may be purchased for 
    less than $5,000. The Institute will monitor activity for 
    purchases for the same supplies, materials, or services 
    from the same vendor so as not to exceed $5,000 per year 
    unless competitively procured as detailed in DOAS 
    regulations and BOR policies and procedures.