Nadie da duros a cuatro pesetas

Nadie da duros a cuatro pesetas
 

In the olden days when the peseta existed, people used to say this saying, meaning nobody gives a 5 euro bill for 4 euros. This expression can be used when somebody sends you an email trying to persuade you that if you send it back with all your details you might get a cheque of 1000 euros and so on.
How can we say that in English.?

There's no such thing as a free lunch.

Thanks

También se dice :"Nadie da cinco por cuatro"

Saludos.