75 bucks is too little money to make any significant hardware upgrade within your build. Then again, your build is well balanced and doesn't need any hardware upgrade.
But if you're running out of storage space then for $75 you could go with additional HDD, e.g this Hitachi Ultrastar 2TB HDD,
pcpp: https://pcpartpicker.com/product/6c2kcf/hitachi-internal-hard-drive-0f14685
Though, if i were you, i'd collect some more money and buy something better along the line (e.g GTX 1070 Ti / RTX 2060).