The $10-20K estimate provided in the first response sounds like in the ballpark, depending primarily on how fancy you go, and especially on whether you leave in the existing roof joists and fit the windows in between them, or try to take them out, which means a significant roof structure modification.
Another thought - if you are primarily looking at more light and a bit of a more "open" look, have you considered between-joist skylight units ? These are designed to fit between the roof joists, and basically only require your roofer or window specialist cutting a hole in the roof, installing the unit with appropriate flashing and sealant, and you are done.
Type and cost depends on size, whether they are fixed or opening type, have blinds or not, energy efficiency, and whether their depth is only that of the roof framing system (4-8 inches), or if you are going to need a "light box" or "light tunnel", which is an opening leadingdown from the skylight to the finished opening interior wall/ceiling, to carry the light down from a recessed skylight.
The drop-in units come in both flat and dome-shaped configurations, and up to 4 feet in length do not get too expensive - a $200-600 for a 2 x 4 foot unit depending on energy efficiency and whether fixed or opening Bigger units get pretty pricey.
Installed cost for a non-opening drop-in by a roofer can run $800-2000, with additional $500-1000 per additional units on same job. Can run 50-100% more if you go with a double-width unit, as it requires removal of part of a roof joist, with associated structural modification to the roof framing.
One consideration is if you are in a deep snow area with winter-long snowpack on the roof, is putting in a "raised curb" under the unit (blocking-up of the roof surface before installing the skylight), both so the skylight is less often buried in snow, and to reduce the risk of ice damming and leakage from the snow melting around and on top of the window frame.
Also, as someone working in a high snowfall area, I would emphatically recommend doubled-up perimeter framing if you go with a double-wide unit (probably an extra $100-150), as cutting the hole for the unit removes a fair piece of the roof's snow load carrying capacity. Ditto in hurricane areas, but for wind uplift load instead.