This is a tricky question;
Let's divide it into 3 simple sections and analyze each of them briefly.
Design & Planning Cost:
I believe you can save roughly 25% -50% on architectural / design costs when using container modules. Although the possibilities are endless when thinking of container placement, the models are all the same size so even at 3000 square foot you're only using 18 modules.
One of the most important factors on any job is time. Lead time, build time, and down time are all important to the building process and can greatly improve or destroy the flow of the project. A traditional "stick-built" structure is typically drawn by an architect and approved by the customer. The materials are then delivered to the job site (often coming in multiple loads through the duration of the project), and the building is constructed. Best case scenario, a small in-plant office is completed-start to finish-in about a month's time. Over two weeks of that time is spent on-site, hindering business and creating dust in the air.
This will depend on location.
What will mostly affect the cost are the following things:
Cold or Warm Climate?
This will affect the insulation and then affect pricing greatly.
It also affects pricing on windows and doors as they also need to be of higher quality and with better energy efficiency.
Are you using used or new containers?
This can double your initial costs, however, can reduce the overall final costs, as you can never be 100% sure of the structural integrity of used containers.
Is this in a commercial area?
If yes, you may be required by Government laws to have a certain type of cladding or facade which can be as or more expensive as traditional construction.
Has your local contractor ever built a office container?
No: You're in for a treat, the first projects are usually disasters, except a fair amount of training is done off-site before.
Yes: Good, within a few hours, you should have your modules placed and interior work can start shortly.
I believe that we save roughly 20% -50% on final constructions costs to our clients, number one factor being location or climate however, I do not believe it is appropriate for everyone and you have a lot of hurdles to jump through to get them approved.
Fully controlled manufacturing environment
More cost effective
Less prone to on-site errors
Much quicker on-site building
Limited on design (kind of) Slightly longer delays to start building.
Due to approval, shipping and manufacturing time.
Once approved, it's hard to change small / specific things