We enlarged two windows. We paid a mason to cut the concrete about $700 - he did a terrible job and we had to clean it up. We DIYed the rest but the windows were only longer not wider. We had to rebuild the frame and put in the new window. You have to make sure structurally as PP said it is possible as it depends on your joists, the header and more. The window could easily be $2000+ and labor at least a few thousand.

I agree with 21:28. Our big bay was going to be $8 to $9K, without having to cut any brick (replacing a window of comparable size). Granted, that was a big one -- 12 feet wide, floor to ceiling. (It was going to have to be craned in -- something you may have to allow for depending on weight and access.)

Before you get too far in, though, here's another question, especially for a sit-in window: will your structure support the weight? We ended up unable to do ours, because the existing header wasn't sufficient to cover the whole span (and replacing the header would've been expensive enough to just knock the house down and rebuild). In the end, we had to leave the old vertical studs in place and put new replacement windows inside them.

Do the engineering homework before spending too much time on the rest of the plan.

the estimates are way low. Bay window of any quality is likely to be 3-5k; Double that to get a reasonable install price for the window. So know you are 6-10 - and 6 is low. Then you have to design/fabricate/install/finish bench. I think you are looking at 15k.
The other challenge will be is that its too small for a big company and too big for two carpenters.
Someone has to design window/trim and the bench. Its more time consuming then you think and alot of the folks in the trades won't spend the time because there's not enough money in the job.

Find a decent design build outfit. But I'd budget 15k before I called them.

OP here --thanks. Yes, they would need to cut into brick. Anybody who has done this? Any more info appreciated.

Far more than that. I would guess $3000-7000. Especially if they are cutting into brick. The window will cost $1000 alone at least. Bench is probably another $1000. 2-3 days of labor on top of that.

1 k for labor, including pickup of all supplies from the store and cleanup after the project. 3 days of work.

I would like to replace our standard dining-room window with a wider and taller bay one, and then build a seating area in it. If you have done this, how much did you spend? How long did it take?

Thanks! FWIW, we are in Arlington.