Where should I host my membership videos?

If you are hosting a large amount of videos like it sounds like you will be doing, S3 is the best option.

I personally haven't used it but from what I've heard it definitely is on of the best options for people in your situation.

But how expensive does it get? I tried to look at their pricing chart but it's hard to figure out where my usage will fall in.

Wistia is great

I was just thinking if I use the "unlisted" feature on youtube for the free membership part and s3 for the paid mini series that would cut the cost down alot, right?

If people pass around the links to the unlisted videos I don't really care plus it would be good publicity and make people want to sign up to my blog.

The main thing to consider is do you really care about people sharing the links to your valuable content? If you do the forgot about YouTube or most hosting services. It's trivial to extract the URL of the video from jwplayer/flowplayer as the video is playing.

So yo need a solution where the URL changes constantly and/or requires some kind of password to access.

There are basically two sensible options, host the video on your own site (outside the area your web browsers can access directly) and have software that serves the video without allowing direct access to the file (which is how most "protected download" systems work) or do as many people have suggested and use a secure streaming service like Amazon S3 (or Amazon CloudFront).

When you use a streaming service, software on your website contacts Amazon to generate a generate a unique time limited (or using CloudFront, IP limited too) URL that ceases to work again once the video has started. Meaning that if somebody does extract the URL from the player, it's only work after a few seconds.

People are often worried about the cost of using pay per Mb services for download, but Amazon S3 works out very very cheap.

Oh, and I didn't want to promote our stuff here, but you mention you're looking at doing this as part of a membership site, well as far as I know only one WordPress membership plugin support secures streaming of this type out of the box and that's the one linked in my signature.