how much does it cost per square meter to restore/

if by reseal you mean spreading a cement mortar to cover the slightly damaged one that already exists, 200/250 euros per sqm - if instead the floor shows deep lesions, it will be necessary to demolish it completely and make a new one, in reinforced concrete, with a waterproof and insulating sheath, to do this it is necessary to have a scaffolding to access and protect the workers, and certainly ask the municipality for a reduced building permit, called SCIA, all this will have a cost of around 800 euros per sqm.

It really is an open ended question.  Price will be affected by factors such as size of roof, ease of access, type of 'sealing' required etc etc.

I'd suggest you seek a couple (at least) of estimates from local tradesmen,  and ask around for any recommendations.