I often find myself keeping
the value page open in a tab when I'm playing DF. On that page we find that Windows do indeed have the same base value as Statues, 25. I don't think Glass can be made into Statues, however.
No matter. As far as furniture made from (essentially) nothing, Green Glass Windows are the bee's knees. Keep in mind that they will detract from the room the value of floor Engravings that they cover. And if they are up against one or more walls they'll detract the value of the wall Engraving as well, if they happen to be facing into the room in the first place, of course. Statues work the same way.
Now if you weren't talking about furniture, but just raw value, then Goblets would be the thing. Goblets have a value of 10, but you make three from each bag of sand. So that's 30, and that's the best way to raise overall Created Wealth for your fortress with little more than 'time' as an ongoing expense.
Unless, that is, you consider the Weapon Trap option. With one Mechanism (which will take a stone, of course), you can pack ten Trap Component weapons onto one tile. Each Trap Component has a base value of 10, so the math goes like this:
(10 (Trap Component weapons) x 10 (Trap Component base value) x 2 (Green Glass value multiplier) x 12 (Masterful value multiplier) ) + Mechanism value = 2,400 + Mechanism value.
On top of that, you could decorate each of those eleven components with Cut Green Glass for up to 2,640 additional value. That's 5,040 + Mechanism value all on the same tile, effectively costing only time and a single stone.
If you have sand and magma, you may find it useful to have a whole slew of Magma Glass Furnaces cranking out Raw Green Glass that you can use to level up a Gem Cutter and, more importantly, a Gem Setter.
By the way, it takes Pearlash to make Crystal Glass, too.
I'm currently on a map with sand and magma and lots and lots of Rock Crystal. It's the first time I've had such a map so I'm learning a lot about glass. (The map also has flux, an underground spring, cave spiders, soil and trees, bauxite-by-caravan, HFS, and goblinite four times a year. It's the richest map I've ever played.)