The argument of those who pay more get a better service has been gone over and over again, and in some respects your right, those who are happy to part with more cash are going to get a better service, but this is hardly exclusive to Thorpe Park or even the Theme Park industry as a whole.
Whilst you are right, £5 per ride is a fair amount of money, the packages offer much better value for money, if anything its the packages prices they could do with increasing IMO.