Kmart is always a cheap option.
As for diy, I guess face paint is an easy option to jazz up an all black outfit – a skull, Vampire and the like.

A white sheet for an angel (or ghost); red face paint for blood and zombie look with old clothes and more red paint

Check out my you tube video, Obviously a few of them cost money but there are 2 in general that you could easily do with a box and an ipad and some speakers.. Its a walking boombox, or a walking karaoke machine. Both are featured in my video Hope this helps!
VIDEO: https://www.youtube.com/watch?v=fIim7dPzmNc&t=2s

Best to look on this site as there are so many great ideas!

why not go in to spotlight or lincraft and get some cheap material and make your own red riding hood or zombie costume with white material?

To be honest, the shops are full of cheap $10 costumes at the moment that I’d rather buy than make. Probably cheaper over all too.

if you have a ols sheet of his toddler bed if he is now in a big boys bed.. cut out holes for his eyes..Ghost in the making or big w have some cheap wears.. If buying a costume i always buy in a size to big so they can wear them again next year..I also buy them off aliexpress from overseas.. but they take forever to deliver.. plan two months in advance for this sight.. and always buy in larger size.. as for some reason aussie kid are bigger than the sizes they state.. They are also good for day to day dresses etc..MY kids are to old for dressups now but when they were younger for clothes etc.. they were fantastic..

Kmart have some silly dressup ideas such as a donut and hotdog. One year I dressed up my younger brother like a girl too and we got a few laughs.

I found these ideas very cute https://mouthsofmums.com.au/10-halloween-costumes-that-wont-scare-your-kids/

Have you seen this one here in Mouths of Mums? https://mouthsofmums.com.au/art-craft/halloween-costume-snugglepot-and-cuddlepie/#ZgcZu8PFtOb1FoRy.97

Face paints can be awesome in making a costume come to life – lifts a basic costume. Tutorials online on face painting are easy to follow too.

Some great answers here already, I think I’ll try a couple. I saw a chicken costume the other day that looked amazing

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.

Hard to go past a ghost with white sheets – easy and fuss free.