The advertising slot are 125x125 and will be on all pages I was going to say £25 for a month £500 for a year.
25£ a month
500£ a year
I guess you adjusted those initial figures, as 25£ * 12months == 300£

Page Rank 1 is quite low, but it is not everything. It is just an indicator for how "high" you rank on Google. Keep in mind, that the PR (Page Rank), can depend on which subdomain you use, e.g., site.yourdomain.com, and even with or without "www", but also if you have several "index" files (or params), e.g.,: index.php and index.php?site=home, can also have 2 different Page Ranks.
About the visits (are these unique, or plain visits from e.g., 300 unique visitors? Do they include search bots?), how much of these are estimated to be automated attacks and background noise? (In my case it's variable between 5% and 15%)
There's also things such as, where are your visitors located in the world? Where are they coming from? (Google? Other sites?)
Not sure which statistical tool you use, but I use Google Analytics for some of my stats, even though this is not 100% accurate even though it gives a lot of nice demographics to use. (Not everyone has it enabled, some has it blocked.)
What do you think is this a good price or is it too cheap/expensive ? any advice would be amazing thanks
Well, it may just be a little bit too expensive, unless you already have a lot of people who wants to advertise on your website.
As some of the other people said here, you could use Google Ads, but be careful. There's a limitation on when you can withdraw money, and more importantly, they may deem your website illegal when it is time to withdraw your money, and then you will lose everything you've earned through advertising. (I've tried that once, and yes they do say in their Terms of Use, at least they did once, that all types of hacking related websites are "banned" from using this service.)
Just my 2 cents, hope I gave you some nice information
