At this point, utility scale solar projects have reached scale in the supply chain, and cost estimates tend to be quite accurate once you break ground and start construction. Yes items such as solar panels tend to be procured upfront and costs locked in (so even if a 2020 style supply chain crisis hits, the extra cost tends to pass down the chain... even though, of course, that sort of crisis will always balloon costs.) The main risk on cost slippage in this sort of project would come from 2 areas: 1) pre-breaking ground (i.e. pre Final Investment Decision)... going through regulatory/licensing/permitting. This is exactly when you can't lock down long-lead time items and are subject to market prices. 2) the transmission line: unlike utility-scale power plants, these tend to be one-off projects and also face a lot of social challenges once construction starts. That price/time could definitely balloon, even after breaking ground/FID.