Cost to Build a Deck

thanks for sharing your knowledge Bond.
The window, that would need to become a door, does in fact have a radiator in front of it which would have to be moved or removed.
The first floor has an extension on building, 2nd floor has nothing (upper duplex of first floor) and thus third floor(our potential place) would have the house extension under deck but 1 floor down. If we end up placing an offer would make it contigent upon approval of deck plan supposedly secured by current owner. Scary territory for someone such as myself with no knowledge in this area. Thanks to brownstoner for easing the pain!

That’s low, but not outrageously so assuming that there are no stairs involved, which is where a lot of the cost comes in and plans have already been drawn up for the deck (I think you mentioned in a previous post that this was the case?). This is one of those cases where I wouldn’t touch this with a ten foot pole without engineer approved plans, but once you have those, it’s pretty straight forward.
You want to try and figure out if there are any hidden surprises before you get started. What’s below where the deck would be going? Is there any kind of structure on the first, or second floor now, and how accessible is the backyard? Would the supporting system be posts or brackets and if posts, you’ll probably need to pour footings, who’ll be responsible for removing the fill? Is there any plumbing or electrical running under the window that you would be taking out? This will all affect the cost. Obviously you could spend more, but the main thing I’ve found when dealing with metal fabricators is that they work much better when they have very clear cut plans in front of them. What seperates good fabricators from mediocre ones isn’t the actual fabrication, but the design, most people in the field know how to do a decent weld.
The key to doing it cheaply, and ending up with a product that you are happy with (IMO) is to eliminate wiggle room. Here are the plans, here’s a material and fastener list, here’s how I want it finished, you’re responsible for clean-up and debris removal, no deviations, build it! The more prepared you are, the smoother the whole process will be. There are plenty of fabricators out there who can handle anything you throw at them, but they are going to charge for that knowledge. If you can eliminate that step of the process, you can save some real cash.
You might also want to talk to the units below you and see if they have any interest in building as well, which would help spread some of the cost. One set of footings could service three units as easily as one, each unit could support the one above it, the workers could start at the bottom and have a work platform for each unit above etc. If you invite the neighbors to build at the same time as you, even if they decline, they’ll feel like they’re part of the process and will probably be more tolerant of construction.