Private servers are made to make profit also. I was surprised to see that some priv. servers has actually more expensive knight cash than official kinght online. People babashop there too.
Basicly you should be aware of two thing: Its either you spend more time (lvling / farming) to catch up with other players (time=money too), or you spend more money to catch up. Choice is yours of course, but because its much easier to earn money in real life, and then spend it on ko than other way around many people choose second option nowdays, not only in ko but in other mmorpg games too.
Advantage of official knight online is that when you quit you can sell your stuff and loose nothing, or even earn. While with priv server, items often get wiped or whole servers wiped or /closed. So you end up with nothing.
If you really wants to spend 0$ and have a lot of time go for it. If you dont have neither money nor time best option would be to find trustable friend - sharer so u can play on his char and spend 0$, and need not to build whole char by yourself.