If you want some really high quality prints try http://www.loxleycolour.com
. Their quality is superb and their customer service is second to none.
However if you're concerned about prices at Photobox, then you'll be getting a shock and a second mortgage to use with Loxley.
Used them many times for my top images/prints & competitions. I use DSColour for normal printing.
If you use Smugmug Pro for photo website hosting, then you may already be aware that Loxley have teamed up with Smugmug for printing from a Smugmu account.