I've been retraining so not on a massive budget (and in debt
) but need to be smartly dressed for work. About £30 a month, less I think.
I tend to go for very, very classic plain cuts even if it's a cheap brand? Thus far seem to have got away with it, even get compliments 
Budget aside, I aesthetically LOVE this style.
So I have two long sleeved, knee length, black fitted dress from George (VERY plain, some sort of synthethic) which cost £5 each. I duplicated as I could see it was a classic style.
And then I match it with a (fake -£2 from BHF charity shop) pearl necklace, black ballet flats for office wear.
Or add a little square neckerchief if I don't want to wear a necklace (very cheap from charity shop, get the small ones in a colour to flatter my face - who needs Hermes?)
I can even wear the same dress for nights out too!
I save money on grooming by having a short wash and go cut (do it myself) and nails are natural and short, I love the look on others but don't love it on myself.
I try to pay attention to posture (shoulders back and down, head up to sky) and do lots of exercise as I think that's probably quite a big part of me feeling confident.