#Java, 283 bytes

    static void p(String p,String l){int[]d={50290000,41400000,78340000,550390000,646270000,1448950000,1627450000};String q="MeVeEaMaJuSaUr";int w=q.indexOf(p.substring(0,2))/2,e=q.indexOf(l.substring(0,2))/2,h=0,m=1,t=e;if(w>e){e=w;w=t;m=-1;}for(;e--!=w;)h+=d[e];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){
		int[]d={50290000,41400000,78340000,550390000,646270000,1448950000,1627450000};
		String q="MeVeEaMaJuSaUr";
		int w=q.indexOf(p.substring(0,2))/2,
		  e=q.indexOf(l.substring(0,2))/2,
		  h=0,
		  m=1,
		  t=e;
		if(w>e){
			e=w;
			w=t;
			m=-1;
		}
		for(;e--!=w;)
			h+=d[e];
		System.out.print(h*m);
	}