For some time now I've traded on Intrade after some successes on the IEM. Recently, Intrade announced a new fee structure - instead of paying a few cents per trade, one has free trading but your account is charged $5 every month or $60 a year (see also the forum announcement).
Initially, this didn't seem so bad to me, but then I compared the annual cost of this fee to my trading stake, ~$200. I would have to earn a return of 30% just to cover the fee! (This is also pointed out by many in the forum thread above.)
I don't trade very often since I think I'm best at spotting mispricings over the long-term (the CA Proposition 19 contract (WP) being a case in point; despite being ultimately correct, I could have been mauled by some of the spikes if I had tried only short-term trades). If this fee had been in place since I joined, I would be down by $30 or $40.
I'm confident that I can earn a good return like 10 or 20%, but I can't do >30% without taking tremendous risks and wiping myself out. So this new fee structure means that I am basically going to do a little trading in January, since I've already been charged the first $5, and then will cash out at the end.
And more generally, assuming that this isn't raiding accounts* as a prelude to shutting down (as a number of forumers claim), Intrade is no longer useful for LessWrongers as it is heavily penalizing small long-term bets like the ones we are usually concerned with - bets intended to be educational or informative. It may be time to investigate other prediction markets like Betfair, or just resign ourselves to non-monetary/play-money sites like PredictionBook.com.
* When I submitted my withdrawal request for my balance, I received an email offering to instead set my account to 'inactive' status such that I could not trade but would not be charged the fee; if I wanted to trade, I would simply be charged that month's $5. I declined the offer, but I couldn't help wonder - why didn't they simply set all accounts to 'inactive' and then let people opt in to the new fee structure? Or at least set 'inactive' all accounts which have not engaged in any transactions within X months?