The cost of Victoria has already been costed as being more - the real issue isn't the cost, but diversion of the main eastern leg of HS2 which will extend the journey times to Leeds and beyond. There are also the issues of how you link Victoria to other rail services, which have been convieniently pushed aside by Sheffield City Council - quite simply, Victoria has been a fools errend and may yet show that Sheffield has shot itself in the foot and not pushed for direct funding for the much needed tram and rail infrastructure to link he HS2 station at Meadowhall the rest of the City Region.
You've missed the point that a spur from Meadowhall to Midland for NPR services would cost money as well. Not only that, but there would be extra works involved at Leeds to handle Sheffield NPR services in addition to the HS2 services. Even if NPR services went into the classic station, they would still take up paths which will be needed for other services, and so more money will need to be spent to accommodate them.
The alternative is that the Sheffield-Leeds NPR service is folded entirely into the HS2 service, which really wouldn't be a problem whatseover. The London trains would have fewer and fewer passengers as they head north, since the reason for the eastern arm being as it is is that Leeds passengers alone won't fill the London trains and to make it worthwhile, Sheffield and East Midlands passengers are also needed. As the trains head north, the number of people on board will decrease, without any decrease in operational cost. If the Sheffield-Leeds passengers can be put on these trains instead the total number of passengers wouldn't decrease so much without any increase in operational costs, thereby making the whole system more efficient.
Even if there is a minor journey time cost in calling at Victoria the question is whether the economic gain from stopping there, carrying more passengers and providing a better transport link is enough to outweigh the marginal loss to Leeds passengers going south of Sheffield. In all likelihood, it probably is, and with the Clegg Kink the difference in journey time between Victoria and Meadowhall has reduced from what it was originally when the two routes were drawn up by Arup.
Even if the London trains will still be too full to take NPR passengers there will still be 2tph to Birmingham from Leeds, and these trains will be effectively empty. In effect, it would be these Birmingham trains which would be the main NPR service between Leeds and Sheffield, with the added advantage that they also link the 'Northern Powerhouse' to both the West and East Midlands, which isn't going to be a bad idea by any means. Victoria wouldn't just serve Sheffield-Leeds but Sheffield-York-NE as well as any other places that HS2 classic-compatibles may be extended to.
All in all, it's an extraordinarily effective way of delivering massively improved rail services between places in the North with minimal upfront cost and a reduction in operational cost. The fact that the only part of the entire HS2 route which is still not confirmed in one way or another is the South Yorkshire section does really indicate that the powers that be are seeing the same things that I'm describing. We know what's going to happen in Leeds, we know that Toton is the site of the East Midlands Hub, we know that Manchester is now sorted, we know that a Crewe Hub is going to happen but we still don't know about Sheffield.
I'm hopeful that my statement above is wrong, and that indeed the folly of Victoria is a bargaining chip for providing both Meadowhall and Classic Compatable services directly into midland station.
I'm well aware of the current issues of joining and splitting - however, we are talking a largely closed system where external influences such as Pacers, trespass and signals faults can be reduced significantly - splitting and joining is a key part of IEP services and will be with HS2.
The more classic-compatible services are there, the higher the likelihood that a problem on the classic rail network will cause an issue on the HS2 network. Any delay could mean that the classic-compatible train can't reach its path on HS2, and if anything goes wrong the scope to sort things out is incredibly limited. The risk of importing delays onto the network is managed by the fact that there will be very few classic-compatible services - Scotland, Liverpool and Newcastle - only to places which absolutely require them. If HS2 Ltd wants to accept more risk in order to allow more places to be served (as well as buying an extra trainset, remember) it will serve places in order of their business case, and there's no way that Chesterfield would come ahead of another possible CC service like Stoke or Middlesbrough or Hull or North Wales or wherever else.