I agree with JOH. In fact, it sounds like the design is the most important aspect to keep right now, since people are willing to donate for the project on the basis of how it looks. I wonder if you can continue to encourage individual donations by inscribing the names of people who donate more than Q.1000 in bricks or something.

My husband and I just started building a house on a small parcel that he owns outside of Antigua. The architect is a friend of my husband's, and he put together the cost "estimate" for us (incidentally, it was also a lot higher than we were expecting). I think that it is typical for architects to do this sort of work in Guatemala because they have access to things like spreadsheets, whereas albañiles usually don't.

Also, since most construction in Guatemala is done "piece-by-piece" and people purchase materials in phases as money becomes available, the whole concept of holding a contractor to a bidded estimate just doesn't work.