# Java, <s>274</s> <s>272</s> 264 bytes (includes Pluto!)

<!-- language-all: lang-java -->

      void p(String p,String l){String q="MeVeEaMaJuSaUrNePl";int w=q.indexOf(p.substring(0,2))/2,e=q.indexOf(l.substring(0,2))/2,m=1,t=e,d[]={5029,4140,7834,55039,64627,144895,162745,140538};long h=0;if(w>e){e=w;w=t;m=-1;}for(;e-->w;)h+=d[e]*1e4;System.out.print(h*m);}

Input/Output:

    p("Mercury","Mars") --> 170030000
    p("Mars","Mercury") --> -170030000
    p("Earth","Earth")  --> 0

Spaced and tabbed:

    void p(String p,String l){
        String q="MeVeEaMaJuSaUrNePl";
        int w=q.indexOf(p.substring(0,2))/2,
          e=q.indexOf(l.substring(0,2))/2,
          m=1,
          t=e,
          d[]={5029,4140,7834,55039,64627,144895,162745,140538};
        long h=0;
        if(w>e){
            e=w;
            w=t;
            m=-1;
        }
        for(;e-->w;)
            h+=d[e]*1e4;
        System.out.print(h*m);
    }