Commercial roofing costs in Southern California vary from contractor to specialist.
A roof replacement or repair cost will be different than a new installation as well.
Since it is a commercial building,
I will assume that it is a flat roof with no pitch?
The biggest thing about SoCal is the heat.
So you will need a roof than will withstand the constant sun damage.
Before you call around for an estimate,
make sure all the roofers you call can produce a Certificate of Insurance.
Read that policy and make sure it is currently active and in force.
you don`t want just a cheap roof as it will turn out to be the most expensive in the long run after you run into problems with it.
look for quality,
craftsmanship and reputation from bonded contractors before you focus only on price.
Make sure they are a member of the roofing contractors association of southern California as well.
Poly foam is good on flat roofs because of its heat reflection ability,
so look into that product.
Depending on the size of your building,
you can pretty much expect that your roof replacement will cost about $20K to 30K if you want it to last 20