MikroTik

https://wiki.mikrotik.com/wiki/SwOS/CRS3xx#RSTP

short: 40G - 1; 10G - 2; 1G - 4; 100M - 10; 10M - 100
long: 40G - 500; 10G - 2000; 1G - 20000; 100M - 200000; 10M - 2000000

What is the port cost for a 2 x 10G LAG group?

2 or 2000 for each link depending on path cost mode. I'm using the long mode since I've got some Netgear switches which are hardcoded with the longer format.

See attached image for my 2 x 10GB DAC LAG between Mikrotik switches.