What do you mean when you say you "have a chopbox".?
When I hear someone say that, I envision one of those old wooden hard maple boxes with a backsaw that gave you a close approximation of an angle you were trying to get.
Pretty much only good enough for rough carpentry.
Unless you can lock in an ACCURATE 45º angle and repeat that cut every time, then the overall look of your work will suffer with gaps in the corners showing and in the long run, it really IS cheaper to just buy chops from a place like American Frames or some other online frame company.
I own my own framing business so I have a Pistorious double mitered chop saw with both blades locked to 45º and I can stand there all day and chop metal or wood and get consistent cuts. That setup cost around $8,000 long ago.
Also like someone said, to cut your own frames, you will need to buy in length, which can only come by freight which can get expensive for what little moulding you might buy. Once the freight charge is added into the cost, your will be close to what you might pay at American frame for their chops delivered...especially adding in the hassle factor of chopping and storage of length moulding.
Most times, it only makes it worth it for me if I buy around 500 - 1,000 foot at a time.
By the way, the specific forum would be http://www.thegrumble.com/index.php