24 August. Paris to Madrid - EasyJet, 59€. 17h30 - 19h45.
25 August. Madrid to Sarria - ALSA, 38,83€. 00h30 - 09h00.
59 + 39 + booking fee + 2 meals + 2 meals + 1 night's accommodation = ~170€

26 August. Sarria to Portomarin. 3 meals + accommodation = 30€
27 August. Portomarin to Palais de rei. 3 meals + accommodation = 30€
28 August. Palais de rei to Arzua. 3 meals + accommodation = 30€
29 August. Arzua to Arca. 3 meals + accommodation = 30€
30 August. Arca to Santiago de Compostela. 3 meals = 20€
= ~150€

30 August. Santiago to Madrid - ALSA, 43,70€. 21h30 - 06h30.
31 August. Madrid to Paris - EasyJet, 103€. 20h45 - 22h55.
44 + 103 + 3 meals = ~170€

170 +150 + 170 = ~500€

Roughly, roughly...

Um... I'll leave it for now. Maybe in late October...