Installing a window into siding is easy enough. It's a bit harder if you're talking about brick.

You can buy basic double hung vinyl windows at Home Depot for about $200. A reasonably talented handyman can do the installation. A window company will charge you a few grand, probably. Look at videos on YouTube to get a sense of the process, which would help you supervise whoever is doing it for you. It's probably one day's work.

It takes a leap of faith to cut through the siding the first time!

It really depends on the house, where the unit is located, etc. I would guess at least $4000-6,000 depending on the window cost. We enlarged two windows in the basement. We paid about $700 (ripoff but that was another issue) for someone to cut the holes larger (we went longer, not wider for structural reasons). The windows were $400 each for vinyl with a few upgrades and we DIY'd the frame, patching the bad cut job, install and trim. It would be a lot more for all the work including where there is no window. We just pulled a permit for a remodel and it was $350 or so.

we are interested too- in adding two windows. we added french doors where a single opaque door previously existed a few years ago and it cost about $12,000.

You will absolutely need a permit to do this.

Well, more like H St, so not historic district. Before we bought our house, the sleeping porches were turned into inside rooms, which is great, except there is one weird room in the upstairs with no window. There is a place for a window and I want to add one. But how expensive is this? Would it damage my house? Does it have to be permitted? My DH thinks adding a window will be simple and straightforward but I'm not so sure. Has anyone done this?