I don't think you can get 3/4" vitreous glass tile at home depot (and if you can I bet they are somewhat limited in colors), though that is where I buy the MDF. I bought a bunch of 1/4" MDF that was already cut down to 2x4 foot sheets.
The tile I buy online, mostly from a place called tileshack.com. They also sell sample boards of all their tile colors and they are extremely useful in deciding what colors to buy. Most online tile places seem to have the same basic color choices, but tileshacks prices seems pretty good to me. They also sell limited lots of vintage tile that have been removed from I guess old houses and some of those are pretty neat. Do *not* buy tile at pearl paint, I went there in a "tile emergency" and was totally shocked at how much more expensive it was than online. It was like 5 dollars for an amount of tile you could buy for less than a dollar online, something really outrageous like that.
It costs about 4-8 bucks for a sheet of 225 glass tiles (depending on the brand and color.)
Sid is 28 tiles by 56 tiles which means he's 1568 tiles, dylan is 27 by 44 tiles which is 1188 tiles (that's actually the first time I did that math and it's a little suprising,I guess I do have a pretty long attention span...) Usually I just estimate that one sheet of 225 tiles is enough for each color and 2 or three sheets for background colors and things like that. I get a bit greedy when I'm ordering tile, and I like to have lots so I don't usually run out.
Since each tile costs less than 2 cents, each mosaic only takes about 30 bucks worth of tile. The MDF is cheap, maybe 6 dollars for a 2x4ft sheet -- I don't totally remember, but that's close. The glue is weldbond and the basic container size will do more than one mosaic and grout is super cheap, a few dollas for a half-pound. I generally estimate that each mosaic contains about 50 dollars worth of materials, which really isn't that bad (since they keep me busy for at least a week and I like the results). Of course, it helps if you plan to do more than one, so if you order 225 tiles of one color and don't use it all, you know you'll use it eventually.