are the cost savings with NPPL (if any) sufficient to outweight the limitations
That must be entirely subjective.
I would say that cost savings are going to be minimal if one is to achieve a comparable degree of currency and competence.
The biggest incentive (in terms of # of pilots who have done the NPPL) appears to be the medical. According to press reports, about 2/3 are previous PPL holders who fail their CAA Class 2 medical. In that respect the NPPL is a good thing, because IMHO the aviation medical requirements are way over the top.
The way to achieve long term cost savings in this game is to fly something which is cheap to operate, which means fuel economy, pilot maintenance, ability to install non-certified parts, ability to use a friendly trusted and competent freelance engineer to do (or help you do) maintenance.
The cost of the initial training and the license maintenance is not significant in the great scheme of things, unless one is flying very few hours.