You can get expensive looking pieces in Primark, H&M, TK Maxx and the like, but you do need to be prepared to look at a lot of stuff to find them.
Dressing for your shape, subtle make up, finding time to go through your current wardrobe and weed out anything unsuitable or past its best cost nothing.
As PPs have said, not too many clashing colours and block colour rather than pattern is a good start.
I think delicate jewellery looks more expensive, even if it is not precious metal, providing your skin is not allergic. I have to wear real gold or silver earrings, as I react to nickel, but have some lovely delicate studs that I've had for some years, and they did not cost a lot.
Maybe you already have some clothes, shoes, jewellery or bags that you could wear more until you find extra pieces to work with them.
I'm trying to be helpful and hope this doesn't come across as patronising. 