Empty legs are a lucrative double-dip for the operator. The original chartering customer has already paid for the positioning flights and crew costs ( inc. accommodation and fees ) as part of the contract.
So when an operator advertises "up to 70% cheaper!" for an empty leg they actually mean "we could sell this leg for $1 and it'd still be pure profit".
There's no reason they couldn't price it the same as or cheaper than an airline flight between two similar points, and feed into a GDS for sale, except that would dilute the image of the product.