"ie can you hire anyone?"
Sure
"or do they have to be approved by the building generally?"
Its always a good idea to check the building specs for sound control. All of the condo work I was involved in used 1/4" cork underlayment (Naples-Ft.Myers area)
Costs? Low end? I suppose you could go with a [url=http://www.hardwoodinstaller.com/hardwoodinstaller/br111_516_engineered_hardwood_floors.htm]
BR111 engineeered[/url] for around $ 3.05 per square foot floated over the cork. Cork underlayment could run from $1.00 to $ 1.50 ---add another .50 to $ 1.00 to glue it down. Check condo specs if the cork can be floated--reduce labor costs. Installation labor for the hardwood could range from $ 1.75 up to $ 3.50 depending on who does the work.
So where does that lead us to? $ 7.30 per square foot if you take the mid road. There will be taxes on materials.
High end? I've seen some expensive ones is my time. Using an example of the
Bruce Studio B on our site that sells in the $8.00 square foot range you would need to add adhesive which can run around $ 1.50 square foot for both the cork and hardwood.
Final tally here would be $13- $15.00
but I've mentioned
our material prices. Retailer cost could add on another 30-50% There are other options, but these two come to mind.