When you mention sales offices/corporate offices I assume you're referring to the big, mass production builders? Have fun. I worked on staff as a designer for a firm that serviced those builders, and later as a freelancer for a different firm. These builders tend to want the front yards at cost or close to it, and if they have a closing coming up you have to drop what you're doing and bang out the landscaping so they can finalize everything on their end. It all sucks.
If you can hook up with a custom builder it can be a different animal, but not always. I'm working with a builder doing a $3.2 million custom home. The plantings, which are not overdone, just appropriate for a house that size, came in at $35k and she's like "whoa whoa whoa let's not get crazy with the plantings here." Haven't met with her yet to go over revisions but I wouldn't be surprised if we get pared back to maybe $20k. So less than 1% of the home value. Come on.