How far should a CAT5 cable be from a power cable?
There is, in fact, an applicable standard.

National Electrical Safety Code (NESC), Section 320 (B) (2),


  Separations Between Supply and Communication Conduit Systems: Conduit
  systems to be occupied by communication conductors shall be separated
  from conduit systems to be used for supply systems by 75 mm (3 in.) of
  concrete, 100 mm (4 in.) of masonry, or 300 mm (12 in.) of
  well-tamped earth.


Only one of those is practical and affordable for a project on a budget, so I bolded it. You can be 12" apart at the same level, 12" above (power deeper, communication shallower) or a diagonal that works.

Type of conduit makes no difference - only the material separating the conduits changes the spacing required.