...I'm afraid I don't think this is a case of apples for apples.
In recent weeks, for the same house, quotes have varied from Eur 323k to 403k to 609k.
The cost for the structure, in all cases, was identical. The reason for the differences in the end price is all down to spec. So, for any more than 1 quote to make sense, they have to be quoting against an actual specification. Throwing planning drawings at a builder and telling him you want an 'efficient' house isn't worth the paper it's written on, tbh...
You need to, and the quoting builders need to, get into specifics.
For example, current build to A-rating around here is in the 85-105 per sq ft range. Do the 70 sq ft prices above include: driveways, kerbing, treatment plants, landscaping etc etc? If not, then they are not indicative, as the 105 price is inclusive of such detail.
And, fwiw, QS's are quoting completely off-the-wall pricing. Again, a recent example saw the house quoted by builder (by self..) #1 at 316k, and by builder #2 (calc'd by a QS) at Eur 475k. Windows alone, for an allegedly identical spec (triple-glazed alu-clad) ranged from 27k to 39k to 50k, across 3 suppliers.
There is way, way too much vagueness and variance in quoted prices for any of them to be taken as actually indicative of......anything.
No more than the person who told me lately he can build a 3700 sq ft, A-rated, HRV'd, airtight, triple-glazed house for......Eur 240k. Simply, impossible. And people will start such projects, and they will go pear-shaped, and people will get angry/upset/litigous......all because the groundwork - the research - isn't thorough.
If the price is too good to be true: it usually is !!
OP, I think your original builder, at 310k, is working off a formula - I doubt very much if any calculation went into it. And if you didn't get asked any questions about your spec, I'd be suspicious..........
In general, I think you'll find that you will get exactly what you pay for, and not a bit more. Do not expect 100 sq ft quality for 70 sq ft pricing. The country is littered with examples of people who talked themselves into the 'impossible dream' and nearly went nuts in the meantime. For instance: what are the guarantees of performance from your 70 sq ft builder? A builder who thinks that upping insulation spec from 75mm to 100mm in the floors (or similar) as the cure to all ills is just a disaster waiting to happen. If they don't have a stake in the project, then they'll just tell you what you want to hear.........ultimately, they won't be living in it..........and I doubt if they'll be posting you a cheque annually for your fuel, either..........