As well you know Charisma ability effects prices -and also the reputation- I wonder about its ration? I mean what's the difference between CHA 14 and 18 when purchasing an item?

In BG1 I created a char with CHA 18 and IIRC the prices in candlekeep were about 10% less.

Read the instruction booklet

that's not what I'm asking for, of course there's an instruction in manual but there, it says CHARISMA adds +5 to reaction adjustement if 18 (+1 if 13) What I wonder is, how does it effect prices? (for example, +5 reaction causes %20 discount)

Note that a CHA above 20 will not affect item cost IIRC, but let me demonstrate how much cash a high CHA can save at the start of the game:
Taking a fairly expensive item: The Staff of Ryhnn +4
Cost with:
CHA 14 Rep 12 = 22275
CHA 18 Rep 12 = 18900
CHA 20 Rep 12 = 16605
CHA 14 Rep 20 = 15525
CHA 20 Rep 20 = 9990

So with a rep that does not affect cost, but high CHA, you are saving 5670 GP = 25%. With a max rep and low CHA, the saving is 6750 GP = 30% (not sure why it is stated as 50% in the manual), but with max Rep and CHA, you save 12285 GP = %55, so you are always getting the 25% discount for the high CHA.