As someone who has purchased ECC SO-DIMM form factor RAM in the past, it is very much not cheap. (it was basically 4x the cost of ECC DIMM sticks)
Trivia: I think the pinout for DDR3 ECC SO-DIMMs are incompatible with the non-ECC ones!
They're "incompatible" in the sense that the pinouts differ, but an ECC-capable DDR3 machine can use both ECC and non-ECC memory; however, the pinout incompatibility means that a non-ECC-capable machine cannot use ECC RAM (but then, who would want to spend the extra $$ for ECC RAM when their machine doesn't support it anyway?).