Think about what you are really after. Most people just want to find beautiful, original, tattoo designs from a reliable site with plenty of images that they can return to. But how can you tell if the design quality is any good with out handing over some hard earned cash beforehand?
Obviously looking at the site homepage should give you some design examples. But it helps to know a bit more. Check out actual feedback from people who have already handed over that cash and don't just get it from the site itself. They are not going to show anything negative.
Check out the site's online reputation. How many monthly visits does it get? How many customers does it have? Then compare it to other sites that offer similar packages. Ask yourself if you are really interested in the bonuses. If you are an experienced tattoo enthusiast then you probably won't need or read an ebook on basic aftercare or choosing your first tattoo.
Some tattoo design sites have extras that are not just ebooks. Some include additional features such as customized lettering, a tattoo artist directory or a translation widget to translate phrases in to other languages.
Many sites have a community that you can get and share ideas and advice. Of course this is a personal thing but think about what are actually looking for and whether the extras have real value for you.
Finally, don't forget that most reputable sites will offer some form of money-back guarantee, especially membership sites and those selling ebooks of design collections.