where can i get a kids altiar costume for under 50$

What an awesome costume for a kid!  They will look awsome in it!  The whole set is costing about $150.00 and up on most online sites when you buy it brand new.  Here is one site that has the outfit:http://www.cosplaytribe.com It cost $189.99 but it includes all pieces.  Other sites I found had the under tunic for $60.00 and the over lay for even more, which came above the 189.99 price.  I'm sure you are looking for the whole outfit and not just a over piece.  Now, since you are looking for under $50.00, it might be harder to find.  Some suggestions I could give you would be to check out your local craiglist website.  Also, you  may find it at local resale shops.  It's a hit and miss but I hope this gives you some ideas.  I'm attaching some pictures of the outfit you are looking for and what the cost may be in pieces.

This under tunic is from Assassin's Creed and worn by Altair the Assassin. His long sleeve under tunic is a soft gray and made of 100% cotton. I also recommend the Assassin's Creed Altair Sword Replica. Stainless Steel Blade •Unsharpened •Metal guard and pommel •Polymer handle with intricate design.