KNOW WHAT YOU MEAN!
My 5 year dd old went through that phase badly around 3 - is still quite fussy - and my ds (2) also has strong views on what he would like to wear. It's a quandry - isn't it? You want them to look nice / be warm and you don't want other people to think that you as parent must be either dirt poor, mad, cruel and neglectful or have no taste. What I did/still do - which seems to work, but don't know if mine are extreme as your's sounds - is (1) always shop with them - even on-line. ( sometimes I show them on-line and then get it without them.) Choose what you think is nice, suitable whatever and choose 3-6 items. Present the range of choice and ask them to choose. Even if they have picked something else out, force a choice from your selection. Then maybe consider also getting their own original choice as a bonus. If she likes none then try putting them all back nonchalantly and get yourself something instead.. It infuriates my children when I act like something's no big deal. Mummy being really cross also works at other times - just have to judge it. Ultimately they always do choose and think it was their idea, as they had the power to make final choice. I also do this with clothes selection, day to day. DD can wear her -mad' clothes on the understanding that she changes if we can out. Sort of dressing up clothes. A bit of threat of punishment mixed with a bit of bribery / dealing and reason.
DD once was the only child to have her tea at a kids club in Greece totally naked - as she threw a fit when I tried to change wet swimming costume for shorts and top. I was furious! so decided to embarrass her. It worked! She also went to nursery in pyjamas once. I know that age 6 is obviously harder to deal with in that sense. That's where 'choice' and 'deals' come in.
I do allow her to express her original sense of dress, though - currently it is three skirts of varying lengths, one on top of the other. She is warm and presentable (if a little gypsy-like) and people usually just think she's a bit of a creative individual! My problem is trousers (she is a skirt, dress girl, pref pink, purple etc). As long as I reasonably present the case for trousers ( ie we are HIKING, dd) and maybe do'a deal' ( she can wear that frilly top under the coat).
I agree with someone else's comment about not spending too much money, but then again I know EXACTLY what you mean about the expensive top. My husband also bought dd a top from Fat face. Should have been perfect - pink, nice embroidered snowflake on front, hooded, soft etc. No! Neck was too high! She wears it sometimes on the agreement that she can wear it with favourite skirt ( luckily because all her clothes are shades of pink, red, purple, mauve, they 'kind of' go - even if they look a little mad!
What more can you do! The more you fight it, the worst it gets - you need to find the bargaining ground - especially when she is being 'reasonable. Maybe get her more involved, or at least make her think she is. Also try and get clothes all within a colour palette ( if poss!).
Good luck! Please don't get too upset! You are not alone!