Quick and dirty comparison of various Ravelry ads I have run recently. With the exception of Group Forum 2, the other ads are general, linking to Midnightsky Fibers. Something like this, mostly naturally dyed and yarn focused:
I pulled these randomly from ads I have run in the last couple months. The average rate is about the same no matter how much I spend (i.e. I don't get a big difference in recognition currently and get more clicks). That being said, I do get better click through rates by using top performing ads- one or two images per ad space instead of a bunch of rotating ones (which I think you also end up paying more for in regular forum ads since each is a different view).
Group Forum 2 has only been running a few days, but it is super targeted- only groups that deal with patterns or vests, preferably both. The other ads all are in generally related forums, or since they are notebook or marketplace they are not targeted.
I suspect marketplace will continue to perform well for me- people are browsing there to look at stuff to buy or think about buying. Forums tend to get a fair number of people looking but not buying, since they are not searching out the items. Forum banner 2 is doing so well because it is geared towards forums where people are looking for items to buy or like to make vests (keep in mind that you can't have ads in your competitors forums though).
Semi targeted ads. I am testing if it is best to send a link directly to Midnightsky Fibers, to product pages, or to the item available on Ravelry.
To compare click through rates for a similar ad style: similar google campaigns average about .03%, and project wonderful about .02%. Which is pretty terrible since I like to aim for 1% click through- though with google, paying per click makes it not such a big deal. Incidentally, while many big bloggers use project wonderful, I have never had great luck as an advertiser on it. I suspect too many ads (even on super awesome craft sites) and many people reading through RSS feeds to be to blame.
Comparable adwords costs for random knitting related items- you can get many longer terms ("cotton yarn for knitting" etc) for .05, but anything related to handspun is pretty expensive.
Compared to my average cost of $.07 and a CTR above .9%, I think it is pretty obvious why I am running many more Ravelry campaigns right now (but haven't given up on google, adwords is still useful)!