I don't know. The periokoi hoplites you can train in sparta have something like a 10 melee attack and 21 defense, cost 1000ish and take 2 turns to build. The Spartan Royal Guard have something like a 12 melee attack, 23 defense but cost 8000 and take 30 turns to build. Seems like there's a strong incentive to never train the royal guard huh?

I'm thinking 3-4 turns and 2000-3000 would be far more reasonable.
Also, I've used both in battle, and there's a very, very minor difference between the two. It's not like the royal guard massively outperforms the periokoi in battle.
Found out how Funky. Go to XGM, Data, and look for export_descr_unit. Do ctrl+f and search for spartan...you'll find it right under periocoi, there will be mention of spartan royal guard. Find that unit's stats, at the bottom where it says 30 change that to whatever turns you want, 8000 you can change to whatever cost you want. Hope it helps! Was very easy, just saved that file and it worked fine.