It depends on the skill set.
However, the general contractor range for full stack developers is anywhere between $80 and $150 an hour. I'd encourage you to get very clear in the contract what your SLA expectation are though. For example, do you expect that them to respond with 2 hours of a request for assistance? Does this apply 24X7? What exactly are they responsible for? The full stack? Only the database? Only the front-end?