In the graph sharing game, two players share a connected graph $G$ with non-negative weights assigned to the vertices, claiming and collecting the vertices of $G$ one by one, while keeping the set of all claimed vertices connected through the whole game. Each player wants to maximize the total weight of the vertices they have gathered by the end of the game, when the whole $G$ has been claimed. It is proved that for any class $\mathcal{G}$ of graphs with an odd number of vertices and with forbidden subdivision of a fixed graph (e.g., for the class $\mathcal{G}$ of planar graphs with an odd number of vertices), there is a constant $c_{\mathcal{G}}>0$ such that the first player can secure at least the $c_{\mathcal{G}}$ proportion of the total weight of $G$ whenever $G\in\mathcal{G}$. Known examples show that such a constant does no longer exist if any of the two conditions on the class $\mathcal{G}$ (an odd number of vertices or a forbidden subdivision) is removed. The main ingredient in the proof is a new structural result on weighted graphs with a forbidden subdivision.