In my estimating travels, I come across a lot of small jobs. Wall demo here, relocate this there, etc. Largely, these types of jobs are for tenants directly and not through property management. What I find is that most tenants are surprised by the cost of what they want done, and I think it largely stems from not having a full understanding of what goes into their request.
For example, a seemingly simple wall demo requires (usually) the services of an electrician, demo company, drywaller, painter, flooring, acoustical tiling, and perhaps HVAC or fire alarm. What you saw as a few hours of some dudes knocking down a wall just got more complicated and thereby more expensive.
This is what I've come to term "Death by a Thousand Cuts". It's not that the whole job is big, but that it's made up of lots of little pieces and each of those pieces is someone's labor and materials.
I have also tried to take this into account as I look at new jobs. I try to be very cognizant of the affects of one request and see what that request will truly generate. Too often I have been caught not paying attention to all the connected items that are generated and not accounting for them, and they end up coming out of my budget.
I think one of the best parts of this job is trying to think through the entire project in your head beforehand to see where it could go wrong. That thinking needs to continue through the creation of the project schedule as you lay out potential conflicts with deadlines and subcontractors.
If I was talking to anyone planning any kind of renovation (even residential), I would advise them to engage an Architect or Contractor up-front to talk about how their design decisions will affect their desired budget. Use their knowledge and experience to help you think through those "unforseen conditions" which could have been handled or at least discovered at the beginning with some careful thinking and planning.