The varieties of plants you get at a flower show like Tatton are not the cheap, mass produced, common or garden ones you find in garden centres and supermarkets and DIY stores. They are more unusual and usually raised by hand by the nursermen and women selling them and sometimes from seed they have gathered themselves on expéditions to their place of origin or specially bred and selected. That explains the price difference.
Primulas increase quite easily so you could have bought one, grown it on and multiplied it by division when it was big enough and then you'd have free plants. Most plants can be increased by division or cuttings so, as long as you have patience, you can get lots of free plants.
The size, style and budgets of the show gardens at Tatton, Hampton Court, Malvern and so on are very different from the ones at Chelsea and they tend to have themes. The great thing about all the shows is being able to see plants at the peak of their condition and talk to the growers about their cultivation needs so you can determine whether they will be right for your garden or skills level.
Next time, go with propagation in mind and a friend who likes plants and it will be a much more rewarding day out.