I need a table that will map each US zip code to the average price of commercial real estate (office space) in that zip code. It doesn't matter to me if the prices are to rent or to buy. It also doesn't matter if the figure is limited to just office space or if it includes all commercial real estate. What does matter is that the same thing is measured in each zip code, so that I have an apples-to-apples comparison.
I'm the head researcher for a commerical real estate brokerage firm. I don't know of a perfect source for what you are looking for, but here are some proxys you might try
Paid option: Costar http://www.costar.com/ has what some of what you want, but they are a pay-for service. From time to time I've heard of them gifting data access for research purposes. I don't think they cover the entire US, and they only look at CRE above a certain dollar size (something like $5,000)
Free option: Craigslist Scrape craigslist.org for office building listings. The URL's are fairly easy to generate for scrapping; for example, I can download the first 100 office spaces listed in New York at the URL http://newyork.craigslist.org/search/off
If you add "?s=100" to the end of that URL (i.e., "http://newyork.craigslist.org/search/off?s=100"), you get listings 100-199 out of 2500. By incrementally changing the "off?s=x" part, you can get all 2500.
Of course, c-listings will on average be smaller properties, but I bet the coverage is pretty good and the bias will be consistent across geographies.
Update: Scraping all base URL's from the about page
If you use R, here's a way to use rvest() to scrape all of the base URL's from the clist about page:
library(rvest) url <- "http://www.craigslist.org/about/sites" aboutclist <- read_html(url) aboutclist %>% html_nodes(".colmask:nth-child(4) a") %>% html_attr("href")
Will return a list of URL's like:
 "//auburn.craigslist.org/" "//bham.craigslist.org/"  "//dothan.craigslist.org/" "//shoals.craigslist.org/"  "//gadsden.craigslist.org/" "//huntsville.craigslist.org/"  "//mobile.craigslist.org/" "//montgomery.craigslist.org/"