Generally, they are $12.99 and you can see a listing of them on Amazon. You can avoid having to pay shipping and get one at Walmart or some other big name store in the area for the same price. The main issue here is that a lot of the time, they don't make enough of the figures so they go up in price quite a bit.
If you search eBay for example, you will see that figures like Diddy Kong can get as high as $100. What Nintendo does is they make them rare on purpose, then then sellers that have no interest in gaming buy them all to resell them to people that actually want them. It's all going to backfire if Nintendo decides to make it a point to do a second release of each figure.
1 - http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=amiibo&rh=i%3Aaps%2Ck%3Aamiibo
2 - http://www.walmart.com/ip/Mega-Man-Amiibo-Wii-U/41488613