Functional Katana for under 100.00? IMO and in my experience, you are asking for trouble and dissappointment if you spend 100 or less on any sword which you might intend to use to cut anything, even if it is only tatami or saplings. If you are going to do anything with your sword beyond hanging it on a wall for display you should absolutely NOT even look at anything under 100 dollars. This is the least expensive I found;
http://www.hanweishop.com/proddetail.php?prod=SH2073
Their practical Katana should be minimally suitable as well and probably just a bit over 100.00
Even today, and even though they are not used for actual combat anymore, one should look at these as investments and consider spending the money to get the best they can find and afford.
Hanwei makes a decent product from what I've heard.
Here is some more to consider though from my experience buying and breaking some cheaper swords.
Usually, modern sword makers will use relatively shorter stock and weld extra length onto the tang for longer, two hand type swords. Or, they will weld threaded stock onto a half tang so that a pommel can be screwed on, in the case of European type designs.
The steel will generally be of lower quality and, IMO the heat treat may not be as good as it could be....
The steel 1060 was mentioned above. I'm sure this is a suitable steel especially considering that in this modern era any steel you purchase today will be of generally superior quality to that found in the past. I've found that most makers of low to high end swords use 5160 however, including Albion Armourers. There are only slight differences but I personally think they are worth considering, edge holding ability being one of them.
Swords made from 1060 are probably closer to what an actual Samuri or Ninja would have used though.