I have volunteered to create 3D minaret's for my kid's school play. Any ideas on an inexpensive way to create the "onion tops" for the minarets?  We need several, in a variety of sizes --- the largest having perhaps a diameter of 10-15 ft.

If you need them to be 10 - 15 feet in diameter, this must be one large stage.  I'm going to assume you want help with the DOME, but have no problem with the circular base it rests on (a simple 2 X 2 or 2 X 4 structure, covered with cheap paneling or cardboard).  What I would suggest is cutting out 8 - 16 identical "rib" pieces (depending on how large the diameter is), profiling the onion dome shape from heavy (corregated) cardboard, or 1/4 inch plywood, or masonite, etc. and positioning them in a circular pattern, joined together at the top.  Band them together along the profile down to the base and cover with "chicken wire," shaping it into the gentle curves of the mineret.  You may need to link them all together with a wooden substructure if you use plywood or anything with a little weight.  Follow this with a layer of good old paper mache and finally paint them with the grays and golds of minerets or however your set is supposed to look.

Thanks to all!!  These are really great suggestions!!