\documentclass{standalone}
\usepackage{tkz-fct}
\usepackage{tikz}



\begin{document}

    \begin{tikzpicture}[xscale=0.6, yscale=0.4]
        %\repere{-0.5}{12.9}{-0.5}{15.9} 
        %\draw[red] plot[samples=200,domain=-1:2] function {x**2};
        \tkzInit[xmin=0,xmax=30,
         ymin=0,ymax=900,
         xstep=2,ystep=50]
        \tkzAxeX[thick, poslabel=right,label=]
        \tkzAxeY[thick, poslabel=above,label=]
        \tkzDrawX[label={\textit{Nombre de visseuses}},below= -12pt]
        \tkzDrawY[label={\textit{Montant}}, below=-10pt]
        \tkzGrid
        \tkzFct[domain=0:30,color=blue, very thick]{0.73*\x*\x + 7.3*\x + 98}
        \tkzText[above right,color=blue](24,655){$\mathcal{C}_C$}
        \tkzFct[domain=0:30,color=red, very thick]{30*\x}
        \tkzText[above ,color=red](24,750){$\mathcal{C}_R$}
        \tkzFct[domain=0:30,color=orange, very thick]{10*\x}
        \tkzText[above right,color=orange](24,275){$\mathcal{C}_g$}
    \end{tikzpicture}

\end{document}