If you're looking to get your hands on the most powerful water gun to make your friends run for cover, you can expect to pay $20 / £20 as a bare minimum for the privilege (a Super Soaker Hyrda costs roughly that amount, for example). However, top-of-the-line blasters can set you back a fair bit more so we'd expect prices to rise to the region of $30 / £30 at most. Basically, the more powerful your water pistol is, the more expensive it's going to be.
Well, we say 'at most'. There are super-advanced blasters that'll really knock your socks off like the SpyraTwo, but those will put a significant dent in your wallet of $100 / £100 or more.