You can do things one of four ways:
1. Provide the forum for free to everyone except the person (or persons) who actually foot the bill.
2. Provide the forum for free to everyone, but solicit contributions (the Public Television method) and hope that the amount contributed covers the actual cost of the forum. It may or it may not.
3. Charge a fee to access the forum.
4. Provide the forum for free to everyone, but run paid ads to offset the cost.
Method 1 is the way it's been done since day one. Isn't it great to get free stuff (as long as somebody else is paying for it?). Method 2 is iffy... relying on voluntary contributions won't guarantee that the cost to provide the forum would be covered. Maybe not even close to being covered. Method 3 is also iffy... how much do you charge per membership? Charge too much and you don't get enough members signing up to cover your costs. Charge too little and again, you don't cover your costs. And with paid memberships, how do you keep members in line? After all, they've paid to be a member... you can bet they would demand to be allowed to say whatever they wanted to say, because hey... they paid to be here. Moderating would be a nightmare.
Method 4 seems to me at least to be the best choice.