Minimal number of clients for MPLS to be cost effective?

Hi guys

I’m pretty new to routing and I was just wondering about MPLS. I have a customer that has five branches and they want to connect them all up. The way I see it is that they can use normal VPNs over the public Internet, or I could setup a MPLS network for them. I imagine is involves buying bandwidth from an upstream provider and then running a radius server to dish out pppoe sessions?

So my question really is whether or not it’s worth setting this up for 5 customers or should I make this available for ALL my clients?

Thanks for the input.