That's assuming that the DSP is underpowered in the lower models. Most often, the parts are the same or very similar, but bits are turned off or otherwise crippled on the cheaper version, but the development costs remain essentially the same for the entire product range.

When it comes to software like that, the cost is typically spent once in writing the software and they just write it in such a way that they can turn stuff off as needed for the "lower end".

Thus they can charge a premium for it at the high end, but in terms of cost to produce, the added feature does not increase cost.