If you really want to pay big money, you can always grab a ticket for England v Wales at Twickenham:-
Cat A £315.00
Cat B £215.00
Cat C £160.00 / Child - £15.00
Cat D £75.00 / Child - £15.00
The full range of prices for the South Africa game is:-
Cat A £85.00
Cat B £65.00
Cat C £35.00 / Child - £15.00
Cat D £20.00 / Child - £15.00
And for the Samoa game:-
Cat A £60.00
Cat B £45.00
Cat C £35.00 / Child - £7.00
Cat D £15.00 / Child - £7.00
The two games at the Amex will be very one-sided, though. South Africa are currently ranked second in the world, and Samoa are ranked seventh. Their opponents will be qualifiers.