There is no "proper commission on a commercial building lease". You should work with a really competent, professional, commercial real estate broker.
He or she will let you know what will get the job done for you, taking all things into consideration, such as:
1. location, type of property
2. condition of property
3. vacancies in your building
4. vacancy for your type of building in the competing market
5. the costs of marketing your property
Only then will a commission be quoted and you can then try to negotiate with the individual. However, if he or she has done the type of research that it takes to analyze your property, they will not likely budge a great deal from their quote.
Onward and upward Douglas... Rob Baird, CA RE License #544165 (One of the oldest, active licenses in CA) 951 515-5855 Email: firstname.lastname@example.org
May 2, 2011