The following opinion is intended with ALL DUE RESPECT.
I'd be willing to pay around $20 per year, but not much more than that. I'm not too sure about excluding folks that do not submit to CGC, though. Access to these boards can be the deciding factor for some that aren't sure they want their pride and joys slabbed.
Some people might be quite unhappy to pay hundreds of $$$ to join a forum only to pay perhaps hundreds of $$$ more to get their books slabbed, and then have to wait perhaps months for their books to even be viewed.
I, myself, understand that CGC is behind, and I understand the convention season is the main reason why, which is why I'm being [wryly] patient even though I'll unlikely attend any conventions anytime soon. But there would undoubtedly be some impatient people that would not tolerate waiting 3 months after paying for a 40 day turnaround [even with the disclaimer] because of conventions. These people may be even more offended that they would have to pay yet even more money in order receive quicker service, just to learn even that is not a guarentee of a timely turnaround [from what I've read from some members]. This may discourage them from using CGC's services again, which would certainly be within their right. Then they would be ticked off at the fact that they have paid hundreds of $$$ for a membership to a message board they will never use again.
I hope this post was relavent in some way.