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Usually p2p is more expensive than cloud services like MPLS (FR or ATM).
With p2p, you can usually implement the "best" QoS, as you're not dependent on what the cloud vendor supports/provides.
As Paolo noted, the least expensive bandwidth is usually obtainable using the Internet. On the issue of QoS and Internet, although the Internet itself doesn't guarantee bandwidth or support QoS, I've found if you carefully manage your ingress bandwidth, and use QoS on your egress (much as you might do for FR or ATM) you often will obtain performance similar to other WAN clouds technologies, even MPLS.
When it comes to VPN across the Internet, the key technique is avoiding oversubscription, nor sharing a VPN Internet port with "regular" Internet traffic. For example, if two sites have a dedicated Internet T1 you can treat it like a private p2p and it generally will provide similar performance.
For hub-and-spoke, you hub should have a aggregate bandwidth that can absorb all spoke bandwidths without oversubscription. The hub will need to shape for each spoke's bandwidth. Doing this, again performance is much like individual p2p links.
If you need spoke-to-spoke traffic, you can, but its very difficult to manage bandwidths sharing the same Internet port (as Internet bandwidth is often so much less expensive, another Internet port might be an option).
