I can tell you some aprox numbers too. I play this game since 2009, but not all the time in these servers. I came back to GF servers in june 2017, and in order to get an endgame PvE and PvP char i bought in the next year around 200,000 dias (I used to be F2P player back in the days, but now i don't have the time for that, and i like the game so i didn't care spend some money if i could get fun). Some items cost me even 4000 dias only to plus from +16 to +20, and i've never tried plusing anything over +25 (and ofc only ring, neckale, pants, upper body and main weapon over +20).
Things that i saw that work really well were diamonds in AH and items permanently in the IS. That with a fair price for diamonds, periodic updates (preferently more than 1 instance per year) and keeping an eye for the fair play is good enough to keep the players playing and the servers alive (and make something with siege wars would be great, over 8 years in beta lol).
I quit buying dias 4-5 months ago cause Ibergard is a tomb for endgame players. I pretty much play just to kill world bosses and keep filling my bank with proofs of myth, and to register SW against alter guild to someday (maybe 5-6 months from now, getting 13 badges per day) I can finally get ironblood will title, since there are no ingame events to get some badges (yes, new players have to wait around 2 years to have the same dmg bonus than old players... 2 years). I was now waiting a promo to buy some dias and at least get transport runes and maybe some consumables, cause thats all i see attractive in IS right now, but what i've found? Now for the same money you get 25% less dias... so, if all i can do is kill wb solo (zzzzz), put proofs in my bank (probably i will never use them), register empty sw in order to get 1 title after more than 18 months, and GF still wants more money from me, i guess is the moment to just let this game die and go to play minesweeper, is more dynamic and actually free.