what is the typical cost to hook up hvac to double stack of blogdetts from start to finish? thanks

depends on code in your area, but assuming all you need is a canopy hood with no make up air it should not be a big deal for a no-frills system. I would use somebody that specializes in restaurant applications to ensure your system is properly balanced with the entire restaurant. It would also depend if you want stainless backing which I would recommend for cleanliness and the length of the run outside for the fan.