yes there has been a lot about costco on this forum lately, but to answer your questions without having to look allover to find bits and bobs of answers...here are some answers.
Membership is the same price here in the UK as the US - @ 25pounds + vat. (I just came back from the US and that is the lowest priced membership there - they have 4 different catagories now costing around $100 for the next level).
As for american products - it's a mix of US/european - so you may not find the specific items you are looking for, but could ask if they can order or add to their inventory - it never hurts to ask.
One of the nice cheap things we use costco for is film developing. It's around 3p a print depending on quantity. They have also just added to most of the UK locations "order from home" so you can send your order in from your computer and pick up later at Costco which is really nice. Otherwise, they do have 1 hour service too.
We buy a lot of books there since the price is better than anywhere else, except maybe used book stores. Also get shoes/clothing but that can be hit or miss.
Happy shopping
