Your phone is old and you need a new one. You'd be happy as a clam if you could upgrade only one part, but to get the RAM/storage/processor you want, you have to pay for everything. This is why people still build desktop PCs. A concept called Phonebloks takes that modular PC goodness and applies it to smartphones. It's an interesting idea that will probably never, ever come to fruition.
Here's the gist: you buy a phone base that includes the motherboard and enough connective hardware to string all the parts together on one side and mount the screen on the other.