If a startup has VC backing then there is absolutely no reason for the startup to offer less than market salary. If a startup does not have money to pay employees then founders should raise more. Or sell more. Employees are not VCs. Simple. Also look this way: if start up is not offering market salary, then think this way: how that startup is going to attract talent from established companies (whose expertise are needed if that startup wants to become big).
On the other hand, If a startup has no VC backing and you are offered less than market salary you are then you should act as investor / co-founder. That will be definitely less money but it can be a great experience: money is not everything.