Apart from the Whiscombe FAC rifles, I think it would be hard to sell a springer for more than £600 or so, and even then it would most likely have to have a nice walnut stock as standard.
Springers are simple beasts and there's a point of totally diminishing returns, which hits the ceiling at around the cost of a new walnut Prosport. Like someone said earlier, additional spend would have to be on tailoring it to your own fickle tastes. Many customised rifles are not my cup of tea at all, especially those with huge bulky stocks, and the manufacturers get it right quite often. Apart from the fishscale chequering, the TX200HC and ProSport are very hard to fault from a design or quality point of view. If the TX200 had come out in 1980 Venom arms would have closed immediately.
In the end it is just an air-rifle, for the enthusiast to fling a small pill of lead 40 or 60 yards down-range accurately. Taking out a mortgage to pay for a springer is like a fashionista paying £2000 for a pair of shoes. There are limits!