You only pay the fee if you are an advertiser/vendor using Google adsense adverts, i.e. the same adverts we get paid for when people click on them. To simply use the tool to check out the value of Keywords to use in your hubs is totally free and no fee needs to be paid.
In simpler terms, if you are selling a product such as a food blender and you want Adsense to put it on sites like hubpages, then you pay the fee. It does not cost you, a Hubpage author anything to use the tools Google provide so that you can sell this food blender for the vendor through an advert on one of your hubs though.
You DON'T need to pay anything to activate the account. I use it often and have never paid a single cent.