Collecting exotic plants of all kinds was quite a popular pastime for the landed gentry and aristocracy of 18th century England. By this I mean that people were devoting a great deal of time and energy to bringing back prized specimens to grow and cultivate in Britain. Such was the benefit of Britannia's increasing global reach. As you might imagine this was quite an expensive enterprise. Bringing both live plants and seeds from the Caribbean, South and Central America, Australia, and India required a great deal of effort. Live plants need sunlight, soil, and fresh water (something lacking on the open sea) while seeds need to be kept safe from insects and rats as well as requiring proper storage. A gardener named James Lee (1715-1795) wrote a meticulous guide on the subject of proper seed management whilst in transit.

With all this in mind, being able to get one's hands on some exotic specimen from a far flung corner of the earth and cultivating it on one's pleasant British estate was a great way to show off one's means. Pineapples were very much a part of this, as were other exotic fruits like mangoes and oranges. These were the best for displaying wealth as such things require a greenhouse in order to flourish. If you had pineapples then you were telling the world that you had the wealth to obtain and cultivate them. Some people went to absurd lengths to articulate this. The Earl of Dunmore's 1761 greenhouse is a good example of this.

Furthermore, pineapples were in high demand not only from aristocrats but from mariners as well. James Cook and others realized that pineapples were a great source of vitamins and could fend off scurvy. Pineapple patches could be found in harbors around the world in order to supply a good diet to sailors. This boosted demand and therefore the exclusivity.

In terms of cost I haven't anything on the specific price of pineapples but I can offer some insight on the money people spent on exotic flora. In Feb 1821 a visitor to the Duke of Marlborough's Whiteknights Park recorded that the Duke had sunk 40-50 000 pounds into his plant collection and was 10 000 in debt. That's an extreme example but it gives you an idea.

Sources: Chilean Trees and Shrubs: A History of Introduction to the British Isles by William Charles Noble, Hidden Britain by Tom Quinn, Foods that Changed History by Christopher Cumo, Daily Life in 18th-century England by Kristin Olsen

Just as a side note to "why not strawberries", they're not really exotic or exclusive fruit - they grow wild in forests, and cultivated varieties are very easy to grow without greenhouses all over Europe, even in the far north (in Norway, the best strawberries are considered to come from Trøndelag, which is just about 300 km shy of the Arctic circle).

Just to clarify, British currency was, and is, the pound. The Euro was introduced to some member states of the European Union a few years ago.