These tubes are absurdly thin, but extremely light at only 90g for the 26" presta valve versions.
Definitely great to keep as a spare tube. They feel so thin I haven't been game to run them in my everyday wheels.
They also make the supersonics for 700c road tyres. Claimed weight of 50g but actually 60g in the longest valve version:
How much are they?
Prices vary but expect to pay between $15 and $20 per tube