The USGS's data puts a 100 lb hot rolled steel bar at $2.15 in 1944, so very very roughly that's $43/ton or $10,000 per ship saved. A Liberty ship is about 10,000 tons, so you'd save about 220 tons by welding. Apparently the HMS Ark Royal, a British aircraft carrier during WW2, saved 500 tons of its total 22,000 tons by using welding, so about 2.2%.
That said, I imagine the steel saved (in both rivets as well as overlap) was probably the main component. I have no figures for how much faster either was in use or training time. Training time wouldn't even matter much - if one process took 10 days longer but was 10% faster, then it would pay off over about a half a year of work. I suspect that riveting was just slower per steel joined per person-hour (much due to the use of 2 people instead of 1), in addition to the excess material used by overlap required by riveting. That said, ships did not use blind rivets, they used a two-person rivet setup where one person would put in the rivet and the other would hammer it on the other side, but that would still be far easier than welding to teach - it certainly would not require 10 full training days like welding apparently did! I've shown several non-technical people rivet guns as well and they've been able to use them easily. Riveting is very simple and I seem to recall I taught myself how to rivet with a cheap Harbor Freight blind rivet gun in a matter of minutes.
I'm very skeptical that it took more than 10 days to train someone to rivet. Not only could welding be done faster, but unskilled workers could be trained to do it far quicker than they could be trained to rivet - it took just 10 days to train a welder >The Liberty Ships, on the other hand, would be welded.
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