KP is free, you just have to create an account. I use it more for answers to questions or issues I am having with a project. Revelry is also free, until you want some patterns, the. The cost is of the pattern. Revelry is not a forum to ask questions but to find patterns.

Ravelry is excellent, but very different from KP. Ravelry is the world's best source for patterns; KP is a "down home" collection of folks like you and me who are posting questions, finished objects and funnies. Neither are commercial "selling" sites, so I wouldn't say one has lower prices than the other. Only members/posters actually sell stuff.

Ravelry is much better and actually has an active admin and moderators