I'm sure NRG is going to advertise their commitment to sustainable energy to justify their higher rates and hope that their customer base are enviro-wackos willing to pay more for "responsible energy production."
It is more complicated than just the cost to store CO2. Because the CO2 is used to produce additional oil, there is economic benefit. But I haven't found details about that and if the carbon capture and selling is oil price dependent.
When the decision to begin this started in 2010 and progressed the price of oil varied from ~$80~95 per barrel while the design, sizing, etc was worked out over ~5 years. It wasn't until after construction started and investments made that the oil price dropped significantly.
The rule of thumb was that you get an extra two barrels of oil out for every ton of carbon dioxide you put in. Were contracts put in place based on dollar amounts oil sold for at that time? Is the CO2 price sliding with the oil price? With this set up, it could be a money maker for NRG with the shared cost arrangements. It could be a money sink for the oil company. I would like to find out more before I would claim NRG has price spikes for selling electricity.
Nearly all the Texas power companies have different rate plans available. I've been in one for years (not NRG) that is tied to the cost of Natural Gas based upon using it for power generation.