The jump from E+ to a proper Premium Economy product can vary quite a bit in price. There are a number of airlines that offer a Premium Economy product but no guarantees that it will just be a couple hundred dollars extra. Some examples include AF, VS, NH and NZ. There are many more.
Beyond that, UA E+ is what you're stuck with. If you are flying it more than a couple times/year buying the annual E+ pass (or flying enough to get status such that it is free) could be a smart investment.