Architect Office is worth the investment for sure as it take very little marble to level it up here's a breakdown.
14,106 Marble needed to get to level 16, thus providing a 16% reduction in building costs.
Here is where it comes into play though...
level 19 town hall costs 34,915 marble prior to any research or building discounts. by the time you've reached this level of building and with a level 16 architects office it only costs you 29,328 to build.
You make back your investment in a few buildings upgraded, not to mention that if you build it early in your towns life...
I have A.Offices in nearly all of my towns, they are worth it for everyone aside from those members that were already high level prior to these buildings being introduced in the v3.0 patch.
Essentially with researching included you're using 30% less stone in your town that has a level 16 architect on site, that is a huge savings and you will notice it in the long run very, very fast.
In every new town I build I'm sure to drop down an architects office as soon as I can, you just can't get around it in the higher levels of building when it's the norm to need 30K or more stone to perform a single upgrade.