Sailen costs 584 diamonds, between Rael at 524 and Ashriel at 745. Almost half his route is free: 10 of 21 steps. No choice goes above 70, so it's a long route made of medium bills, not a couple of big ones.

Where the diamonds go

The route starts in Episode 2, The Glided Cage, with the free "I'll play along." and "What a punishment… Fine." for 44. Then:

Episode 3: "New World style" and "Lead the way." — 70 each, 140 for one episode. The first one is a look.
Episode 7, Under the Raven's Wing: "Share another story." — 66.
Episode 9, Confession of a Recluse: "Dig deeper." for 31 and "To Sailen's taste" for 56, plus one free step.
Episode 11: "Cook dinner with Sailen." — 51.
Episode 17, The Dead Triangle: "Stay." for 50, between two free steps.

The latest step is "Sailen" for 49 in Episode 18.

How I'd play it

Episode 3 is the one I'd plan: 140 diamonds, and half of it is an outfit you can skip if you only care about the scenes. The rest is fine to pay as you go. Compare with Rael and Ashriel.

Other love interests

Ashriel745 Rael524

Questions and answers

How many diamonds does Sailen's route cost?

584 diamonds for 11 paid choices, all in Season 1. At 7 ₽ a diamond, that is roughly 4,100 ₽.

What is the most expensive choice for Sailen?

Two choices at 70 in Episode 3, The Price of Obedience: the "New World style" look and "Lead the way."

How many of Sailen's choices are free?

10 of 21. Episode 12 alone has three free steps, and Episode 17 has two.