Considering that skin gifting is now a thing recently allowed, I'm sure gifting champions would be no different, seeing as they both fall under the same feature.
You want to look at the RP cost of whatever you want.
Twitch: 790
Orianna: 880
Jhin: 975
You're probably looking at rates between 1 rp : 0.65-85 fg.
People might round to 0.5 or 1 just for convenience.
If you're getting better rates than that, they're probably desperate for fg.
If you're paying more than that, then that probably means there's more demand than supply.
Lower rates would be more likely for bulk sales, such as 5-10k+ fg, with higher rates for smaller sales.
People seem to be scared to post what they want, and would rather discuss via pm than share the rates they want/offer.
Don't instantly jump at the first person who offers without having anyone or anything to compare with unless you're happy with their offer.
Simply calculate from there. 790 + 880 + 975 = 2645.
Expect to pay between 1720 - 2250 on average for those 3 that you listed, realistically between 1980 - 2250 since it is a smaller amount (+/-10 fg, I round off odd numbers)