In 1840 average weekly wage for a skilled man in his 30s was £2 according to the first link on ddg. A penny post would be about 1/480th of a weekly wage. If we assume that a skilled man in his 30s today would earn £40k, that would put today's stamp in the £1.50 range, rather than 65p.

Far cheaper today, despite being pretty much meaningless. It's no wonder people "hacked" the system, this was a time before phones, or even telegrams, but a time when long distance commerce and connected economies were beginning to become established.