It would be nice to have a Building Calculator tab at the top that uses the last two formulas on the wikia page below. There could be a drop down for which building (or perhaps a separate. static field for all buildings), two fields for the number of buildings you currently have (defaults to zero) and the number of that building you're shooting for. The result field would spit out how many cookies you would need to save from that point on to buy that many of that building.
If the field of how many of that building you already have is changed from the default zero, then the underlying formula would need to change from "- 1) / 0.15" to "- 1.15ᵇ) / -0.15". If that isn't possible to change on the fly within this language, then I guess make it work however you can. lol..
It isn't that hard to enter the formula on a scientific calculator, but anything to make our job a little easier, right? =)
edit: I just stumbled across this earlier. Basically your own version of what this does - http://cookieclicker.wikia.com/wiki/Cookielator.