#Java, 274 bytes (includes Pluto!)

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

      static 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]*10000;System.out.print(h*m);}

Input/Output:

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

Spaced and tabbed:

	static 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]*10000;
		System.out.print(h*m);
	}