I build tons of workshops in XCOM. Then I wondered - what's the point? They reduce the cost of stuff. Then what? Do they reduce cost of buildings too?
The primary purpose of engineers is to meet the minimum requirements needed for the manufactured items, Foundry research items and Satellite Uplink / Nexus building facilities.
All of those have a minimum number of engineers required, and part of the strategic portion of the game is ensuring that you have the correct number of engineers available as the things you want to build/research become available.
After that, extra engineers provide a reduction to the cost of items that are built, as per the calculation and graph available here. Extra engineers do not provide a reduction in the cost of building facilities. Note that there are diminishing returns on the price reduction (the link notes that once you have three times the minimum required engineers, the amount of discount each extra engineer provides is quite small).
Finally, building extra Workshops beside each other for an adjacency bonus is also important, as that returns a portion of the cost of the item / research / facility when it is completed. In Enemy Unknown, each adjacency bonus provides 7% of the cost back, while in Enemy Within, the bonus has been increased to 10%. The Foundry and MEC Labs also count for the adjacency bonus, so ensure they are built beside any Workshops that you build.
Personally I feel that once you've reached the number of engineers required to build your final Satellite Uplink, extra engineers and Workshops are not as relevant to the strategic portion of the game. The money provided by the countries, as well as the loot obtained from the larger UFOs means that you aren't financially strapped, and time is on your side. If you need a bit more money, play another month before starting the end game.
Several items and upgrades require a minimum number of Engineers before they can be built/stored (such as Advanced Construction, which reduces the construction time of buildings). Don't underestimate the discounts that Engineers provide. Elerium and Alien Alloys are very valuable, particularly in the late game when there's a lot of things to spend them on.