1.) there is no fee at all
2.) u can only do that if u plan on buying an ethernet hub which will split your internet connection. i have i and it works great. i can be on my cpu on the web and online with no problem.
3.) since u have cabl their will be no lag but if u play someone with dial up u might have lag so watch out and look at opponents conn. bar before u play them.
4.) i got mine from best buy for 40 bucks and i think it is 40 dollars everywhere. unless u can find it cheaper some where else then i would go with best buy
i hope this helped u