How much extra cost did it add to the project?

I estimate about $15 per square yard extra for the various expendables. For a typical 50 square foot surface area kayak that is approximately $85.

How much weight do you think it saved?

I'm not sure. There should be about 20% less resin, so for the 50 sqft kayak that would be a couple pounds before a fill coat. While I am still using a fill coat, the surface before application of the fill coat is smoother so less epoxy is needed to create a smooth surface, so there may be another pound or so saved. These are guesses, I don't have empirical data yet. Note that the vacuum will likely cause the epoxy to penetrate deeper into the wood, so some weight may be added. However, this may result in a stronger layup.

Compressing the carbon fabric straightens out the fibers which is said to make the layup up to 20% stronger. The infusion resin also has better physical properties than the standard resin for hand layups, so the overall strength should be significantly better with some weigh savings.

Do you think there's any advantage in infusion vs hand layup and vacuum bagging?

Vacuum bagging has just about all the same expendables. The difference is you need to lay down the peel ply, release film, and breather (instead of flow media) immediately after applying the resin. You then need to get the bag sealed up and and relatively leak free before the resin kicks.

With the infusion I could take all afternoon or even a couple days to get all the pieces in place. To my mind infusion is easier. I could spend time making sure all the layers were smooth and well fitted, I didn't have to worry about dripping resin on the bag where I would eventually want to apply seal tape.

I considered vacuum bagging but I was also worried about the low pressure sucking air out of the strips and that air getting trapped in the layup. I don't know if this is something to worry about or not, but it seemed to me that infusion allowed for a relaxed pace.

This did not work out initially when I was using the thinner bagging film, but once I dialed in my methods, the process was actually quite easy and stress free.

I will say that the layup with carbon fiber is much better than any I ever achieved with a hand layup. Chasing and finding bubbles and bridges and starved fabric in a hand is tricky. I can't say I ever had a perfect hand layup with carbon or Kevlar. With the infusion I was able to get a perfect layup by the time I had done it a few times. By perfect layup I mean no bubbles large or small.