yticklabels = {0,num{20000}, num{40000}, num{60000}, num{80000}, num{100000},%
                   num{120000},num{140000},num{160000},num{180000},num{200000}},

documentclass[tikz]{standalone}

definecolor{maincolorLight}{HTML}{838fbc}
definecolor{maincolorMedium}{HTML}{425b9b}
definecolor{maincolorDark}{HTML}{003d80}

usepackage{pgfplots, siunitx}

pgfplotsset{
    Eksamen1/.style={
        scale only axis,
        grid=major,
        grid style={dashed, gray!50},
        axis lines=middle,
        inner axis line style={=>},
        xlabel={large $x$},
        ylabel={large $y$},
        yticklabel style={inner ysep=0pt, anchor=south east},
        xticklabel style={inner xsep=0pt, anchor=north west},
            after end axis/.code={
                path (axis cs:0,0)
                    node [anchor=north west,yshift=-0.075cm] {0}
                    node [anchor=south east,xshift=-0.075cm] {0};
            }
    },
}

begin{document}

    begin{tikzpicture}
        begin{axis}[
            Eksamen1,
            ytick={0,2,...,20},
            yticklabels = {0,num{20000}, num{40000}, num{60000}, num{80000}, num{100000},%
                            num{120000},num{140000},num{160000},num{180000},num{200000}},
            xtick={0,1,...,8},
            ymin=-.1,
            ymax=23,
            xmin=-.1,
            xmax=9,
            domain = 0:8,
        ]
            addplot[color=maincolorMedium!30!white,thick,samples=50] {20-2*x} node[above,pos=1] {$g$};
            addplot[color=maincolorMedium,thick,samples=50] {20*e^(-0.2*x)} node[below,pos=0.5] {$h$};
            addplot[color=maincolorMedium!30!black,thick,samples=50] {20*0.9^x} node[above,pos=0.6] {$f$};
            node[] at (axis cs: 2.5,21) {bilens verdi (kroner)};
            node[] at (axis cs: 7,1.5) {AA r etter 2017};
        end{axis}
    end{tikzpicture}

end{document}