In the end a contract is only as valid as the enforcement behind it.

Between two people of the same nation, a court willing to say "Yeah that's valid" and enforce it with the power of the state makes a contract quite powerful.

Between two entities that cannot agree on a means to arbitration, or have that means enforced on them, it's basically only as valuable as their willingness to accept it.

"A contract is a contract is a contract… but only between Ferengi" might seem like a straight up dismissal of another's species rights to be negotiated with, but its also a warning. If the Ferengi authorities don't have the power and will to enforce your contract with a Klingon living in free space, then a contract lacks the enforcement clauses that make it absolute.

So how binding is it? As binding as the parties allow it to be.