I'll be going. Although only a 6 hour event, Wonder Festival is one of my favorites. Entry is Y2000 if I remember right. That includes a guidebook with map.
As far as cosplay rules are concerned, it's probably the most relaxed large scale event in Japan. Basically there are no rules. Real swords, air soft guns, large over sized costumes, you name it it's good to go. Also it's fun to roam around and check out all the sellers and displays. It's an easy event to navigate; Cosplay area outside, event inside. It takes all of two minutes to go from one area to the other.
One heads up however. Cosplay has become rather popular at Wonder Festival over the past several years. You used to be able to just walk straight into the changing room without a line and the room be half empty. This is no longer the case. Expect a line at the cosplay registration desk and the line for checking-in/dropping off your bags (if you choose to do so) is usually quite long. Getting there early helps avoid the line some. But with the event being a one day thing, might as well be a little early anyway.
There's usually a few foreign cosplayers, but most of the foreigners are there for the event proper and there's usually some foreign press. The event itself is a "garage kit (amateur figurine/model) convention, but there is a commercial section where major companies announce new releases and such which attracts international journalists. That and the amateur figures are usually much higher quality than the professional ones so people who run Japan blogs and such show up.
Since the spirit of Wonder Festival is amateur craftsmanship, cosplay fits in perfectly.