LED Balloons. Use LEDs on a string (available at craft stores and flower specialty stores). Place in balloon, fill balloon, leave string trailing out. Clip bottom of balloon and add a new string, rather than tying. Moments before event remove LED strings which cause LEDs to start working. Guests will be amazed. LED light each $2.25, Balloon each $.80, Small Helium tank rental with waiver approx $75. Total for 50 balloons? Approx. $250.