I think it is fair to say that there is always improvement between PSR models, but seldom massive differences from one model to the next. These changes might be incremental improvement in sound quality, look, feel etc. Seldom are these absolutely groundbreaking, model to model. But these incremental changes add up, and over the span of a few generations can be quite significant. Also sometimes there are some new features introduced which did not exist at all on older models, like touch screen for example. If these individual features are important to you then you have to go to at least the model where they were first introduced.
If you have a psr730, then you will notice a difference going to a S970. It is an outdated model of course, so cannot be bought new, the latest model in this range being the SX 700 or 900. These have touch screen, but would both fall outside your budget. However decent used 970s should be available on your budget.
Mike