As with anywhere else, the cheapest places normally are those affiliated with non-profit groups such as the VFW or KofC.
Assuming you are not timing this event for the part of the week when wedding receptions are normally held, probably the best value in the Orlando Metro area is the Lake Mary Events Center. They are booked way in advance for weddings, but if you are planning a weekday meeting you probably can get a spot. It is a stunningly nice facility for the price.
There are also some very affordable spaces near downtown Kissimmee that are owned by Osceola County and the City of Kissimmee.