Raise them to the highest price you can still sell them. Since people tend to race to commission you when you have commissions opened I think at the least a modest increase is warranted. You could always raise the prices like 60% and then give a discount to those who have watched you for say a year or more or have favorited/commented a lot of work, that would make the commission price at a modest increase from current prices. With a set price for active watchers and one for new watchers it could help stifle the blow for those who like to commission but can't afford a double of your current prices which I think could still sell. Perhaps if they don't sell fast enough doing a special everyonce in awhile where first time commissioners get a 10-20% discount. I know you can still sell commissions if you up them, probably even still sell a lot if you up them a lot, but it looks bad when you have to lower them as well so finding the right price is difficult and if you can find a way to up the base price quite a bit and find a way in rush settings to sell more with it still looking highly sought after is necessary.