In English, we say something is "too good to be true" to express that apparently amazing deals or benefits generally aren't real. Is there an equivalent phrase in Spanish?
The literal translation of it is the common way of saying that:
Demasiado bueno para ser verdad
also you could say something like:
Demasiado bueno para ser cierto
Also there is a proverb for that kind of situations which says (thanks leonbloy and Icarus):
Cuando la limosna es grande hasta el santo desconfía (used at least in Mexico)
De eso tan bueno no dan tanto (used at least in Colombia)