The mistake in your calculation might be based on the bad scaling of that quark mining.
GTX750Ti=640 cores
GTX980=2048 cores
That's 3.2 more cores, so instead of 12000 it should be 19200.
But taking the "quark" stats from Crypto Mining Blog shows
for the GTX750Ti: 5272 khs
and the GTX980Ti: 15249 khs
Whatever goes on there, it's an anomaly of quark mining. Under Hashcat the scaling is pretty normal and the performance of the GTX980 will mostly be 3.2-3.5x over the GTX750Ti since it overclocks very well. The GTX980Ti will be around 4.5x.
But I agree about the prices, although you make the most common mistake nearly everyone here is making too: you compare perf/price ratios per card. That's naive fallacy. Cards don't run alone, they run in a system. The whole system brings the performance. So to run your 18 GTX750Ti you need three systems. The comparable system with GTX980Ti would be 1. Yes, you would always take the more expensive GTX980Ti over the normal GTX980, because you always minimize the number of systems and try to maximize the power. Taking a look at the prices here in Europe that's a no brainer anyway.
But since 6 GTX750Ti cost less then one GTX980Ti and you would need 4 of the latter to compare performance, they might have an mostly equal price/performance ratio. Since handling one system under Hashcat is way less of a hassle comparing with three systems you would always go for one powerful system then three weaker ones.
GTX750Ti=640 cores
GTX980=2048 cores
That's 3.2 more cores, so instead of 12000 it should be 19200.
But taking the "quark" stats from Crypto Mining Blog shows
for the GTX750Ti: 5272 khs
and the GTX980Ti: 15249 khs
Whatever goes on there, it's an anomaly of quark mining. Under Hashcat the scaling is pretty normal and the performance of the GTX980 will mostly be 3.2-3.5x over the GTX750Ti since it overclocks very well. The GTX980Ti will be around 4.5x.
But I agree about the prices, although you make the most common mistake nearly everyone here is making too: you compare perf/price ratios per card. That's naive fallacy. Cards don't run alone, they run in a system. The whole system brings the performance. So to run your 18 GTX750Ti you need three systems. The comparable system with GTX980Ti would be 1. Yes, you would always take the more expensive GTX980Ti over the normal GTX980, because you always minimize the number of systems and try to maximize the power. Taking a look at the prices here in Europe that's a no brainer anyway.
But since 6 GTX750Ti cost less then one GTX980Ti and you would need 4 of the latter to compare performance, they might have an mostly equal price/performance ratio. Since handling one system under Hashcat is way less of a hassle comparing with three systems you would always go for one powerful system then three weaker ones.