If you don't mind wearing used clothes then charity shops have some good bits in them.If you don't fancy that or can't find any clothes you like they always have lovely scarves,jewellery,handbags that you could use to give a bit of a different look to your outfits.
I used to wear skirts and blouses,I shopped at Primark,Tesco,Peacocks,New Look.Tesco usually has some really good sales on,I've bought skirts that were for work from there that were all knocked down to £8 each and they were still going strong 3 years later.
I wore heels,not high one's the court shoes,I used to get them from M&S.
For flats though I'd go for ballerina pumps and loafers normally but now it's getting colder you might want to look for a flat slim ankle boot or some womens brogues,there's some really nice light brogues around now.
Shoe zone are really cheap and usually have lots of different types of shoes that would be suitable for work,Primark would be worth a look as well for shoes for work.
If you can pick up 3 different outfits,a couple of pairs of shoes,3 more tops and a few different accessories(for 3 days of work)you'd be able to make at least 12 different looking outfits and it should be easy to do on £100. 