Im not really sure of where u can find a $50 CAI...you'd have to be pretty lucky. I have the Injen RD series CAI and it works great. One thing you have to realize about the installation tho, the CEL will come on no matter what you do. The only solution to the CEL is by buying a cpe MAFci customizer, but this will run you for another $250 buks.You may be able to find something you need a this site and it provides great info. as well.
http://forum.mazda6tech.com/viewforum.php?f=24 Go through this forum section and look for some items on sale. You may find something you like.