In the simplest form it doesn't cost anything to form a club. A club is nothing more than a place where people can come together to fly their airplanes. So you can do this for no cost at all.
However, there are things that will add to the cost of forming a club.
The biggest cost for most clubs is the cost of the land/facility that they use. In the case of our club we lease 80 acres for the cost of $1,500/year.
Now I know the next thing is going to bring many people out of the woodwork and scream that the sky is falling if you don't do this. But the AMA costs for a club. First of all, you do not have to have AMA to be a club regardless of what some may say. There is no law stating that you club must be an AMA club. But for most clubs out there they want to be part of the AMA. Why?? Very simple, insurance. When a club is chartered through the AMA there is insurance that protects both the club and the land owner in the event an accident happens and legal actions are taken against the club and/or the land owner. This insurance is separate from the individual liability insurance provided to individual AMA members. If you decide to AMA charter your club the costs are $90/year. The fees are $30 for the club charter and $60 for the insurance provided to the club and land owner.
It's not required that you incorporate your club, but many do for legal purposes. If a club is incorporated then all of the members and officers of the club are protected if any legal actions are taken against the club. Many clubs in the US operate without doing this, it's going to be a personal choice made by your club. The fees will vary from state to state so I don't know what it's going to cost you.
It is correct that you have to have 5 AMA members to start an AMA chartered club. It's a very easy process. We formed out club 5 years ago and it's quite easy to do. The best advice I can give is skip asking here in the forums and simply pick up the phone and call the AMA. When we chartered our club I called the AMA and within 15 minutes I had all the info I needed on getting our club up and running. The staff of the AMA is very helpful and they will be able to give you the exact information you need to get your club formed.
Hope all of this helps some.
Ken