I found it at last :
- 1 euro < 15 cm
+ 2,00 euro = 20,25 of 30 cm
+ 1,00 euro > 30 cm
([Price]+ (abs([Size] < 15) * (([Size]-15)/5))
+ (abs([Size] = 20) * ((([Size]-15)/5)*2))
+ (abs([Size] = 25) * ((([Size]-15)/5)*2))
+ (abs([Size] = 30) * ((([Size]-15)/5)*2))
+ (abs([Size] > 30) * ((([Size]-30)/5) + 6))))