How about a funnel and plastic tubing painted silver or gold? 

I've made a couple of props recently that resembled the challenge of making that curved horn shape. One was a big spiral snail shell, and another recent one involved horns on a mask.

The following takes ideas from how I did the modeling of those shapes for a final product in papier-mache. Try this:

Determine how long the horn would be if it were unwound into a straight horn rather than a curvy one. I'm guessing around 3 feet.

Take some pipe cleaners and twist them together to total that length.

Now cut a series of circles from thick cardboard or foam board. The largest of these circles will be close to (but not quite) the size of the wide end of the horn. I'm guessing 4" diameter. The smallest would equal the narrow end of the horn, perhaps 1/2 inch. The progression of sizes in between should be in smaller increments for the narrow end. I'd go with 4",  3",  2", 1-1/2", 1-1/4", 1-1/8", 1", 1", 7/8", 7/8", 3/4", 3/4", 5/8", 5/8", 5/8", 1/2", 1/2", 1/2", 1/2".

Punch holes into the exact centers  of the circles with an awl or narrow screwdriver. Thread the combined pipe cleaners through the circles, in order of size. It's helpful for the holes to be narrow enough for the circles to stick in place when you force the pipe cleaners through. Put all the circles on the wire roughly the same distance apart. If they don't stay in place, use a wad of tape near the center to make them stay in position.

Now carefully bend the structure to the shape of your curved horn, e.g. like a ram's horn, perhaps with a spiral near the narrow end. Make sure the circles follow a somewhat even curve. Place wads of tape between the circles to keep them at the angles you position them in. I've also just taped the circles edge to edge (with a tough tape like duct tape) to make them hold the form.

The end result is to have a fairly evenly-formed horn skeleton on which to layer papier-mache or similar material for the final and smoother surface of the prop. One more thing to add is a funnel, perhaps of cardboard, extending from the largest circle terminating in a slightly wider open circle a few inches further out. This is so that the horn appears hollow on the wide end from the audience perspective.

My favorite materials lately are plaster cloth strips for the first coat (they dry so wonderfully fast) followed by working a layer of paper clay (a smooth, slip-like version of papier-mache which also dries quickly) to make the outer surface as smooth as possible.

Metallic spray paint should complete the illusion. Remember to paint the inside near the wide end of the horn black to conceal the solid middle of the piece.

Someone else recommended using a funnel.  I didn't know they made such large ones.  Yes, this is probably the idea I'd use if I hadn't found an instrument technician to custom make me one!