YOu can print masks off the internet pretty easily, and they can provide the basis for cool but simple costumes.

Definitely open shop shopping. I have found amazing pieces in these stores and Kmart have costumes for $9

Depends what you’re going for but you can do a robot with a cardboard box, if you don’t have one large enough ask around the local shops they often have ones they are throwing out. All you need is the box and some foil and a bit of paint and you can make whatever robot you like. (Think tissue boxes for feet etc)
If you want something quick and easy you can also go as a present and again all you need is the box and some wrapping paper (maybe save some from presents).
A box can also make a great book costume just paint the outside to suit the book you’re going as.

I’m not really the creative type but I have found some great costumes for my daughter at the charity stores for only a few $!

OP SHOPS! They have lots of ready made costume and unlimited supplies to create your own.

Google is great for ideas, I made my kids costumes up myself for book week. Sometime you really only need accessories, depending on what look your after.

I posted these ideas on another post requesting Halloween themed costumes :

A Ghost – all you need is an old bed sheet;
A Mummy – just wear white clothes and wrap toilet paper around, with tomato ketchup for a gory effect;
A Bat – wear all black and cut up a cheap umbrella from the op shop to make wings;
A Cat or a Mouse – wear all one colour, make ears to stick to a headband and a piece of rope or thick string for a tail;
A Pumpkin – check out the throw out boxes at the supermarket for orange cardboard, cut out two circles, tie together at the top and slip over the head;
A Skeleton – paint white bones on some old black clothes;
Zombie – tear strips off some old clothes and use face paint (Spotlight) to paint scars and blisters.

Were you after specific themes – i.e. Halloween, or just costumes in general?