I've already turned in one treasure map, so I have the achievement and I can get into the Blue Building. However, I have no idea what amount of sP/items I got when I turned it in. Is it generally worth it to save up all treasure map pieces to turn in, or is it better to just sell the pieces after turning in the first map?

Thanks for asking this it is just what I was wondering too

You get ~10k sP and 5 rare / super rare items (between rarity 75 and 95). Normally it might be worth it if you get lucky with the items, but because the piece is used in a Genetech combination to make the Treasurefly it has inflated to 3m, meaning it's much better to sell the pieces now.

Thanks at least now I know so will sell the pieces off instead of collecting them..I know only too well about that piece bought one recently .. ouch

This is good to know, thank you for asking and for answering.