You are unreasonable to keep her off school.
I'd suggest contacting the school and making them aware that dress up days are problematic and sugesting that they limit the number of days or try to focus on the children making something to wear themselves.
However in the meantime most themes can be sorted from normal household items and clothing with little or no skill or cost. It doesn't need to be a big deal, but it does require some thought.
I would suggest a post on here or on Facebook asking for free or low budget ideas a week or so before the event. Involve your child too, the point of these events is not about buying something on Amazon it is about the child thinking about the theme and getting involved. They might not be able to make a whole costume but they can come up with ideas and do some making.
To make it easier keep old scruffy clothes adults and children's plus sheets, towels, curtains etc rather than binning them. Also keep a few cardboard boxes from deliveries and any other bits that could be useful. Plain white t-shirts, black t-shirts and black leggings are especially useful.
Start the process as soon as you hear about the theme, don't leave it to the last minute.
For lots of themes you can do stuff with existing clothes. So think about characters that wear normal clothes or variations on normal clothes like sports kit, party clothes, any previous fancy dress costumes etc.
Adding simple items found around the house or made from cardboard or a sheet or using make-up gives you more scope.
Lots of people on here will help with ideas. Some examples:
World Book Day - Charlie from Charlie and the chocolate factory, wear normal clothes and make a "golden ticket" or George from George's marvelous medicine, wear normal clothes take a bottle or a saucepan and spoon.
Victorians - chimney sweep/coal miner/newspaper boy/urchin. Normal dark clothes and a blackened face. If possible add a prop like a shovel/brush/newspaper.
Romans - white sheet knotted and pinned to make a toga wear with sandals.
Egyptian - use the sheet and add eyeliner.
Farming - jeans, wellies, shirt/jumper/fleece = farmer add some mud and a toy animal for added effect. Alternatively wear black trousers and top, a white scarf, make a tail from the leg of some black tights and some ears from card, black eyeliner whiskers = sheepdog. If you don't have a white scarf the same can be used for the farm cat.
Jobs/careers - normal clothes and use a bit of white paper to make a vicar's collar. Normal clothes and a home made warrant card for a detective. Normal clothes and a lanyard can become pretty much anything. Sports kit for a professional sports person or PE teacher.