Lego is expensive in general but Star Wars lego is expensive because of the brand. Like most Star Wars branded stuff the prices always seem to be inflated.
Take for example the lego Christmas calendar. Normal lego one, £19.99, Star Wars Lego one, £24.99. Both have little tots in each day so they are no different, just one is Star Wars.
The sail barge is expensive as it is no longer made. Any piece once discontinued jumps in value. I bought the Jawa Sandcrawler in 2009 when they discontinued it for £80. I sold it this year for £260.