So I own a website (forum) which gets over 10,000 unique users per day, and 150,000 page views.

How much could I expect to make off pay per impression advertising? I can't find any general idea on google... any thoughts?

Are you going to be using a third party advertising network to get the ads? Or are you offering it to advertisers directly? If its the first, then you can expect to make however much the third-party pays. If you are looking to offer direct, and want to get price estimates - it really depends on where the advertisements are and how 'noticeable' they are. I would think that because its a forum, it would be difficult to charge more than $1/CPM (cost per thousand), probably closer to $.50 CPM. You may be able to boost it up to $3-5 CPM - all depends how well you can sell your ads!