It's a double sized issue, hence the double price!
I'm so pleased for them that they achieved this milestone. Like many people here I'm sure, I have every single issue of Doctor Who Weekly/Monthly/Magazine, all bought when they were first issued. It's the only magazine that I still collect.
The magazine did so much to help to keep the flame alive during the "dark days", when interest in the series must have been at an all-time low, but against all the odds it still survived, which is a great example of how resilient and loyal DW's fanbase actually is, no matter how much we may moan about things sometimes!
The magazine isn't cheap, but you get what you pay for, and I've always thought that its content is of the highest quality.
I won't be around when they get to issue 1000 (if I am I'll be 105!) but I hope they make that milestone too!