I know he wasn't stupid. Obviously. I, too, think that in such a situation, if I was on that side, I would think that sacrificing 50 lives was well worth creating a loyal, near-indestructible supermonster to help us.
Yeah, from their point of view, it was worth it. If we considered ubermonks to be a form of military equipment, many different countries in the real world have spent as much money producing effective machines. It is indeed possible.
But it does cause some problems for them. For instance, it would be difficult to
keep making ubermonks if the Elantrians figure out a consistent means of destroying them. Don't forget that the other side has their own form of super-soldier whose limitations are in location rather than expense. Thus a full-fledged assault on a
prepared Elantris may be unfeasible.
Of course, this depends to some extent on the limitations of the supersoldier form that we aren't familiar with in detail. For instance, how do they distrupt the Dor? Is there an upper limit to how much damage they can take before their protection fails? Would it still be possible to destroy Dilaf with a big enough blast that overwhelms his protection?