First, I agree that skill replacement isn't always the best method of handling thaumaturgy difficulty. As stated, if you do a Resources replacement, a twelve shift ritual could likely get you a nuke, which is overpowered. On the other hand, if you wanted to do a thaumaturgy teleportation like spell and used an Athletics replacement, those same twelve shifts only moving you twelve zones seems underpowered... especially when you consider that wouldn't come close to letting you leave the blast area of the nuke that last fictitious ritual just summoned.
However, in this case, I don't think a skill replacement on Resources is that bad of a gauge. Consider what mundane items would be needed to accomplish the same thing, the drawing of a picture on one device that lets it appear on another device. Seems like a pair of electronic tablets with a 3G connections would fit the bill and you could probably buy two for between $500 and $1,000... putting the difficulty at Fair since a Fair Resources roll would let you buy anything under $1,000.
As I said though, that kind of thing doesn't always work. The big thing to ask yourself anytime you set a difficulty on something is, "Does that sound reasonable in terms of cost and benefits?" Here, it certainly sounds reasonable to me.. you're only putting in a Fair effort and in return you're getting something useful, but not amazing. It will allow the characters to communicate silently and effectively across a distance, but it's not going to win any encounter for them all on it's own. Plus, it requires planning ahead before it's actually used, which always makes a difference to me when I set difficulties (Wanna pick the lock on the exit door before the fight starts to make sure you have a quick way out? Burglary roll of two. Wanna do it while under fire and trying desperately to make an escape? Difficulty just went to five.)
