It’s never cost us anything but mine never went dressed as their favourite character necessarily, more just as a character from a book based on what we had. We had a box of superhero costumes that were largely Christmas or birthday gifts then I just used to fashion stuff out of whatever we had. I have no creative bones in my body so I couldn’t actually make anything but various things included.
Victor Frankenstein - old white shirt of DH’s, some red marker, wrote a name badge, Goggles from a science kit we had and my own makeup to make his eyes a wide red and manic.
The Creature - Harry Potter cloak from the dressing up box with a rucksack containing a cushion underneath to create a sort of hunch and then my own makeup to make him a bit more interesting.
Musketeer - Woody from Toy Story’s hat and a drawn on moustache
Poirot - had a bowler hat bought for Halloween and added a drawn on moustache with a waistcoat from a wedding and bow tie.
(My personal favourites were various Sultan’s that they went as but we have South Asian heritage so had outfits worn to weddings with headgear made from my shawls and then draped in lots of my costume jewellery).
I mean, overall I do agree, but as someone with not much money and no artistic streak and a job and study, it was ok.