We known a fine hotel 200 meters from the railway station st. lucia : Hotel Il Mercante di Venezia. The average double room rate ist € 70 / $ 84. Fully air condition (in July it may be very hot in Venice)
You can book online. Klick on the link and you will have some pictures from the hotel too.
We stayed there some times. This is ohne of the cheapeast hotels in Venice and not more expensive than hotels outside (may be Mestre or so).