Katja,
The best answer to the question of costs is that it can cost as little or as much as you want, and it depends on what you want to do and look like. There are people that buy homes and advanced shapes, skins, animations, and clothes. But there are plenty of freebie shapes, skins, animations, and clothes. And you do not need a "home." In fact, one danger is collecting too many freebies in your "inventory" that you can't find what you want when you want it. Janet can provide you more information on what it cost her.
For my example, I'm a taverner outlaw in Gor primarily. I started with a free shape and skin and free clothes. I paid for a bow and arrow and sword. I later found out I could have gotten serviceable weapons for free. The weapons might not have had bells and whistles but they would have done the same damage as paid-for weapons. I still use a free shape, but I upgraded my skin and paid for the skin to look a bit better (from what I can see there are far more free options in skin for women than men). I also ended up paying for an outlaw outfit, but actually my current mix and match outfit is mostly free and could be all free if I changed the pants which is the only thing remaining from the outfit that I bought.
Yes, there are some people that buy, buy, buy, but as I say it is entirely unnecessary and there are generally free options around. There are even websites around that guide people to free and nearly free items.
http://themedinsl.blogspot.com/The biggest issue is time. You find yourself on and on and on.
Besides guides you can find from googling, you can also find tutorials on how to do things on youtube. Or, the best way to learn is to ask others while you are on line.