Kits can be very cost-effective, especially when there are a lot of different fabrics involved and you may have to spend a lot of time hunting down all the fabrics. They also allow you get unique quilts in many cases.
You should always look at a kit as being a starting point. If you want to modify the kit in any way, then you may need to purchase additional fabrics. We have a lot of customers who will buy a lap quilt kit and then expand it to a full or queen size quilt by adding borders or more blocks.
Always check out the full product detail description before buying a kit. Some places offer precut pieces and these kits, while they save you labor and time, are normally more expensive, because of the waste and additional labor involved in the piece cutting process.
I don't know about other stores, but on our kits we always include the pattern unless it's a commercial pattern that a lot of people might already have. Then we may offer an option to delete it and the amount charged for the pattern. Again, check the product detail description and kit options.
Many of our kits are made from free fabric manufacturers' patterns, so in that case we're not charging for the pattern, or for the time we take to proof it, so there really isn't a pattern charge when that's the case.
If the quilt is taken from a book or magazine, we will normally include the book or magazine price in the kit price and include them. Selling a kit taken from a book or other commercial pattern is, IMO unethical unless the buyer has already purchased the pattern or book from the designer or through another outlet.
We always cut each piece generously so you don't get shorted on amounts. Maybe it's because we're quilters that we do that, but we'd rather you have some left over than be shorted. We typically add a bit to the amount shown on the pattern when we cut, unless we know the designer to be generous in their estimates. Designers vary a lot on their fabric estimates for their quilts, so cutting exactly to the pattern may lead to shorts in kits when the designer is stingy with their estimates. I doubt many shops that sell kits would intentionally short the customer on fabric.
Hope this helps!