Please see this:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml#bestpath

Although you can certainly manipulate the route's metric to influence the routing decision, BGP does provide other, more preferred methods for route control. For instance, metric, or MED, is actually the 6th value examined in the routing decision process. Its far more common to manipulate the weight or local preference to influence a routing decision since its close to the top and will have little or no chance of being overridden by something higher in the decision process.

Also, just wanted to make sure you understood that BGP does NOT use metric the same way a IGP, like OSPF, does. For bgp, in the traditional sense, the metric is merely a differentiator when you have two direct connections (as in AS connections) to a remote AS.

As for an example, if it were me, I would do something like the following:

ip prefix-list branchx permit 10.x.x.x/24

!

route-map local-pref permit 10

match ip add prefix branchx

set local-pref 200

route-map local-pref permit 20

!

router bgp yyyyy

neighbor zz.zz.zz.zz route-map local-pref in

!

Alternativly, you could also just set the weight, it really depends on your topology, your downstream neighbors, if any, and how many routers your trying to manipulate.

HTH

GL

-Rob