I assume you are referring to PPS-Sieve.
Which Nvidia cards?
My 2 280x cards, one in a mac one in a windows box, were slightly faster and/or had better unit throughput (in last years challenge) than a 680, 770, and 980, but all of those were in macs.
It is slower than my 1060 (.21 hours for the 280x, .17 for the 1060).
The 1060 is going to use half the power (or less) of the 280x.
IMHO, unless you are running GFN21 or 22, anything tahiti-based is going to be rather expensive to operate for its output (I have 8 tahiti cores running, and a very painful electric bill along with them).