I went through this about 1 year ago with TG in their office in Silom Road, Bangkok. The Thai policy was to charge 10% of the 'published' fare. I couldn't get a definite answer which fare bucket that was but it did not seem to be the full fare Y. It was much more than 10% of the fare we were flying on at the time. This was the case for both Economy (where I had purchased my ticket) and Business (which I had upgraded to). In our case, my infant son had to pay 10% of the 'published' business fare.

US$442 for PNH > BKK > MAD seems expensive for Economy. IIRC, it cost B16000 (US$400) for BKK > LHR return in Business a year ago but as uastarflyer says, it could be due to taxes, surcharges, etc.

TG tried to tell me this was an IATA rule and no airline charged 10% of the lowest fare available but when I pushed them to show me that rule, they couldn't find it...

I also got the feeling that if full-fare Y was the only seat available, that is the one they'd have charged 10% of. Is it possible that one of your 4 legs is sold out so full-fare Y is the only fare available?

Edit to add:

Unfortunately, in TG's case, there doesn't seem to be a good source of reliable information on this. The website indicates that it is 10% of the full fare. When I called Royal Orchid Plus, they told me that our son could fly free (+ taxes) because I was on an award ticket. The only way to find out for sure is when they are issuing the ticket...