In A Theory of Justice, why would those in the original position not trade basic political rights and freedoms for a higher standard of living?

I think that Rawls does believe that individuals would not
trade off basic political rights and freedoms for a higher standard of living.  I think
that this is so because of his use of the veil of ignorance.  In this configuration,
individuals would not necessarily know where they would fall into the social fabric. 
This would mean that individuals would design political policy with the lowest of
society in mind for this would be them.  If this is true, then they would not trade off
the intangible and intrinsic value of political rights because individuals have already
understood that fleeting value of these entitlements.  For Rawls, the protection of
individual rights is the basis for all other rights, including the right to property and
material wealth.  It is because of this that he sees individuals as not being entirely
susceptible of trading off their political entitlements for upward social and economic
mobility.