Most office supply stores (Staples, OfficeMax), copy shops (UPS Store, Kinko's) can make B./W double sided copies with 1 or 2 folds. We have used these for years for our smaller productions' playbills. Many of them can make color posters also, but a true print shop would likely be cheaper for a large run of full color posters.

Don't forget you need to buy the rights to do the show from the copyright owner. Additional fees are required for most music. The fees are usually due several weeks before the show opens and are based on the number of seats available (even if you don't sell out). The fees are required even if the show is for a charity and even if you don't charge an admission fee.

The box office services (selling, returns, refunds, will call, credit cards, on-line sales, checks) could easily be 10% of the ticket cost.

Renting a nice stage for the show and rehersals is not cheap.

Equipment and technicians to provide lighting effects and sound (wireless mics for actors) is not cheap.

Many amateur groups hire a professional director... Again, not cheap but often well worth it.