You could model it roughly as adding the capex cost of half a turbine or so and x% of uptimes to your solar/wind capacity. Lifetime would also be longer but anything after 2060 or so is largely irrelevent for slowing climate change so we'll ignore it.

IIRC O&M + Capex for a CCGT is about $20/MWh and fuel is around $30 which would put the capital around $5 and we don't expect to operate it for long as it's a backup.

So the generator should cost us <$10 per MWh of solar + wind we use (with the majority being used directly or via batteries/hydro). Green fuel will cost about 2x the renewable energy with capex/O&M on the same order as the turbine or maybe a bit lower (ammonia electrolyzers are estimated about $1000/kw with current tech but you'd only need to put half or less of your peak capacity through it. Additionally they will run most of the time producing fuel if the target is using the fuel as the last resort backup).

With projected solar + wind costs around $20/MWh using chemical storage for winter would then have a total cost of $60/MWh for energy that has been stored as ammonia and $40 for energy that has not in a somewhat pessimistic ballpark with price dominated by fuel generation. On par with some current full time gas, but at the upper end. Far less than fission if we use the same 'joules before 2060' constraint, but enough to cause difficulty building out new gas turbines if we don't presently have enough.

Fusion isn't really in the running as a steam turbine is much more expensive than a gas one so we're getting close to parity with the renewable + battery + gas turbine just from the steam turbine step.

Until enough renewable generation is built out, it will be silly to build storage, so the turbines will continue burning NG, just increasingly mainly at night and on dark, calm winter days. As their duty cycle declines, their total annual operating cost falls in proportion. As chemical synthesis and compressed or liquified air storage gets built out, later, the fraction of their (reduced) operating time they spend driven by those instead of NG increases. As non-synfuel storage capacity (battery, pumped hydro, mineshaft gravitic, buoyancy) increases, running time and maintenance load declines further, as those pick up more load. Synfuel and compressed air will be mixed into NG in proportion increasing with stock on hand, rather than running on one fuel for a while and then another. Eventually NG falls out of the mix.

Operating the turbines only when the renewables and non-fuel storage are not supplying enough power cuts the maintenance load, therefore operating cost per unit calendar time. Not running does not mean you are not getting revenue: revenue comes in for the renewable-generated power, produced at near-zero marginal cost.

Running a turbine only sometimes extends its total life, so its capital cost is amortized over just as many kWh produced, either way, just longer in, again, calendar time.