I am going to guess that you have a two-node MAN, since you only mention a corporate office and the remote office; and that your MAN topology is a simple point-to-point and not a ring, since you're looking into redundancy. Since you've already ruled out redundant MAN (ring or diverse-routed point-to-point connection) due to cost, I assume that your company has decided that the cost of downtime and mean-time-to-repair is acceptable.
Chances are, that telco circuit you're thinking about using for backup also runs along at least part of the same pathway as the MAN fiber. So if something catastrophic happens that cuts the fiber, like an 18-wheeler hits a pole, odds are it's going to take out that telco circuit too. If you want to guarantee survivability, you have to make sure your backup media takes a different pathway so that there's no single point of failure.
What's your geography like? How far apart are the two locations? If you have direct line-of-sight between them, consider free-space optics (lasers) or wireless (microwave). If there's something simple in the way, sometimes a tower on the roof of a building (or even a tall pole) plus a directional antenna is all you need to get over or around local obstacles to make the connection. I have a health-care system customer that uses wireless microwave for backup links between two hospitals; and a school district that uses wireless as their primary data and voice connection between all their buildings, with leased lines for backup.
If you don't have line-of-sight, geosynchronous-orbit satellite service is an option. It's not as "far out" as it sounds; sometimes it's the only way you can get a connection at all. A couple of years ago, though, I was involved in a WAN feasibility study for a customer, where we determined that outside of a leased T1 point-to-point star topology, the next least expensive solution for them was to build private (dark) fiber. Line-of-sight microwave or laser was not possible due to the terrain, and satellite was the most expensive. Maybe pricing has changed in the meantime, I don't know. But if you can afford to buy satellite time, you can probably afford to build more fiber too.
Hope this helps.
