The oil leak is what causes the gasket to fail.
It's coming from the rocker cover gasket and has nothing to do with the pressure drop.
$300 will cover the cost of the studs and the cost to remove the studs but you'll still have to find someone willing to remove the turbo an manifold. $600 is normal for that job
The gasket costs $110, the studs and nuts are $50, turbo gaskets are $70 and you are looking at 4 hours labour plus whatever time to extract the broken studs. It's not an easy job and you will be very lucky to find someone willing to do it for $300. And if you do find someone, it is highly likely it will be a learning curve for the both of you.