ausdr = 500 - s + 0.05/0.00025 Log[s/490];
wurzel[i0_,rho_] := x /. FindRoot[x == Exp[-(500-x)/rho] (500-i0),{x,250}];
abb = Plot3D[wurzel[i0,rho],{i0,0,250},{rho,25,500},
AxesLabel->{"i0","b/a","s_unendl"},ImageSize->{400,300}];