bgp path metric

[Rohrmann Sascha]

Hi,

I have 2 routers which perform an iBGP peer.
Now, every route, which they exchange should get a special metric (e.g: 
received route from same ASN, route metric= route metric + 100).
How am I be able to influence that?
I've already tried to check from which AS a route came and preprent (I am not 
sure if this is the correct keyword btw) it.
Here's my filter settings so far:

export filter {
                if bgp_path ~ [= ASN =] then
                               {
                                               bgp.path.prepend (ASN,100);
                                               accept;
                               }
                };

I'd be thankful for every help.