How much extra disk space does txindex=1 require? For example for a 1 MB block, does the extra Tx index database take up another 100 kB on average?

And a follow-up: once my node has completed -reindex after setting txindex=1 can I COPY the blockchain AND the TX index database together to other nodes? I know the blockchain can be copied but I'm not sure if the new node needs to reindex the blockchain itself or if the index database copies over as well.
The total extra size is around 7GB as of November 2016. You can check by looking at the size of the $DATADIR/blocks/index directory.
If you copy that directory, you'll copy the index with it.

Updated values from comments:

11GB as of 2017-05-08 – via
ens
21GB as of
2019-07-16 – via Vasil
Toshkov
27GB as of 2020-06-14. – via
Anonymous