First I'd try keeping your current door, but sealing off all the cracks. Add a weather-stripped threshold to the bottom and weather-stripping gaskets around the door frame. This will get you the most improvement for the least amount of money.
The next best reasonably priced step would be to fill your current hollow core door full of sand. Cavity-filling foam is another option, more expensive and less effective, but lighter and also less messy.
The next best reasonably priced step would be to fill your current hollow core door full of sand. Cavity-filling foam is another option, more expensive and less effective, but lighter and also less messy.
