Symmetric Economic Curves

Is it better now -- i have also corrected the label W1 -- it was misaligned

begin{tikzpicture}[scale=0.5]
    draw[thick] (0,11) node[left]{$P$}--(0,0)--(12,0) node[below]{$Y$};
    draw[thick] (0,-11) node[left]{$L$}--(0,0)--(12,0);
    draw[thick] (-11,0) node[left]{$W/P$}--(0,0)--(12,0);
    
    draw (1,8) to [out=280,in=175] (8,1);
    node [right] at (8,1) {$AD(M_0)$};
    draw (3,8) to [out=280,in=175] (8,3);
    node [right] at (8,3) {$AD(M_1)$};
    draw(5,0)--(5,9)node[right]{$AS$};
    
    node[left] at (0,4.2){$P_1$};
    draw[dotted](0,4.2)--(5,4.2);
    
    node[left] at (0,1.9){$P_1$};
    draw[dotted](0,1.9)--(5,1.9);
    
    draw (-1,8) to [out=260,in=15] (-8,1);
    node [left] at (-8,1) {$W_0$};
    draw (-3,8) to  [out=260,in=15](-8,3);
    node [below left] at (-8,3) {$W_1$};
    draw(5,0)--(5,9)node[right]{$AS$};
    
    
    
    
    foreach Point/PointLabel in {(5,6)/A, (6,5)/B}
    draw[fill=black] Point circle (0.05) node[above] {$PointLabel$};
end{tikzpicture}

If you want to mirror something, you can also invoke the magic mirr— ...ehm, I mean a scope and apply in your case xscale=-1.

Then you don't need to calculate anything. Of course whether this approach is convenient or not depends on your case, but for your question is a possible solution.

By the way, I have fixed some of your commands that were redundant, for example you can attach a node to a draw without having to explicitly write it as a separate one.

Output

Code

documentclass[tikz, margin=10pt]{standalone}
begin{document}
begin{tikzpicture}[scale=0.5]
    draw[thick] (0,11) node[left]{$P$}--(0,0)--(12,0) node[below]{$Y$};
    draw[thick] (0,-11) node[left]{$L$}--(0,0)--(12,0);
    draw[thick] (-11,0) node[left]{$W/P$}--(0,0)--(12,0);
    
    draw (1,8) to [out=280,in=175] (8,1)
        node[right] {$AD(M_0)$};
    draw (3,8) to [out=280,in=175] (8,3)
        node[right] {$AD(M_1)$};
    draw(5,0)--(5,9)node[right]{$AS$};

    draw[dotted] (0,4.2) -- (5,4.2)
        node[at start, left] {$P_1$};       
    draw[dotted] (0,1.9) -- (5,1.9)
        node[at start, left] {$P_1$};
    
    % MAGIC MIRROR
    begin{scope}[xscale=-1]
    draw (1,8) to [out=280,in=175] (8,1)
        node[left] {$W_0$};
    draw (3,8) to [out=280,in=175] (8,3)
        node[left] {$W_1$};
    end{scope}
      
    foreach Point/PointLabel in {(5,6)/A, (6,5)/B}
    draw[fill=black] Point circle (0.05) node[above] {$PointLabel$};
    end{tikzpicture}
end{document}