While you're waiting for the wise ones to digest their xmas pud, here's my view...
The SJPdP-Irun idea may not work at all at the end of March. If the weather is anything like as wet and as cold as it was this year you will really struggle and I think it will take longer. Even though this is the easier and lower part of the GR10 it is generally more demanding than the camino. I've not walked the part west of St Jean, but looking at my guide book I think it will be similarly testing to the stages I've walked from St Jean to Lescun. Honestly, given the time you have I'd save it for another trip and concentrate on doing justice to the Norte.
But if you are going to stick to your idea my suggestion is as follows. Working back from Santiago, 11 walking days with viable places to stay averages 31k pd - if you start in Gijon. It's a lovely city but without an albergue. The youth hostel is a bus ride out from the centre and is modern but lacking any magic. I'd suggest finding somewhere in town, but check the Gijon calendar as it's a bit of a festa town and you may find Easter gets booked out early. Tia mentions Oviedo; be aware that it's a bit of a diversion off the typical Norte route.