In reply to GStone:
To walk from the Mer de Glace to the Grand Montets mid-station would be a nightmare. Unless you took some very circuitous route, you would be dealing with very steep, difficult moraine banks, and potential danger from stonefall. Frankly, there is only one sensible way to make this journey: go back to Chamonix, go up the valley to Argentiere, then take the cable car.
By the way, if you are planning to go to the Argentiere hut, it is an incredible slog from the mid station. Much better to go to the top, and then descend, but NB this involves crevassed slopes, for which you need to be roped.