Just looking at trade topics it seems like
1m RS3 = 3fg ( 1fg = +/- 325k ) roughly
1m OSRS = 30-32fg
So for 1.3B RS3 , I'd expect to get around 120M-130M OSRS. You can do the exact math if you want to.
A lot of RWT sites , which I don't know if it's kosher to mention specifically, have pretty easy to digest tables or scales you can use to find the swap rate.
This is because unlike jsp, these sellers sell a set amount of ingame items/gold for a set amount of USD.
I.e. if a certain amount of currency (or fg) gets you 500m RS3, but only 40m OSRS on the same platform , then you've found your swap rate. Just divide 500m/40m and it's a multiple of 12.5x between the two games.
This is just an example, use live-market numbers (check the day of trading) to find a fair multiple to give your friend.
This post was edited by Keasbey_Nights on Mar 3 2022 07:47pm