
This was one of the Great Debates of the GW2 RP community (not just GW2RP, but-- the RP community of GW2) and I'm sure it's only harder to suss out in FFXIV, just on account of its... well... SE-ness.
Personally I don't have an IRL money comparison to make. When it comes to the infrequency IC gil usually comes up, consistency isn't terribly important, really- so most of the time you can just say "a little bit of gil", "a lot of gil", or "the appropriate amount of gil and a generous tip" or something like that, but if an exact number is necessary, I compare to the closest NPC equivalent I can find- armor vendors, food vendors, guildleve quest rewards, etc.
Personally I don't have an IRL money comparison to make. When it comes to the infrequency IC gil usually comes up, consistency isn't terribly important, really- so most of the time you can just say "a little bit of gil", "a lot of gil", or "the appropriate amount of gil and a generous tip" or something like that, but if an exact number is necessary, I compare to the closest NPC equivalent I can find- armor vendors, food vendors, guildleve quest rewards, etc.