From Diablo Wiki
→NPC Item Generation
Higher level characters see more Exceptional and Elite items offered for sale.
Exceptional: (<100*(Ilvl-Qlvl)/2>+1)/100 % Elite: (<100*(Ilvl-Qlvl)/4>+1)/100 %
Which works out to about:
Exceptional: (Ilvl-Qlvl)/2 % Elite: (Ilvl-Qlvl)/4 %
In this formula, Ilvl = (Clvl - 5 + Rnd<10>), Rnd<10> is 0 to 9. Qlvl is the level of the base item picked (that is of the exceptional/elite item we are checking to upgrade to).