Not quite sure that I understood what you were asking, sorry if the following is off the mark...
A pair of 12" open baffle servos (recommended 2 or 3 drivers per baffle/per amp) will be 740 before shipping or cabinet materials, alone. The two 8's, alone, are probably not what you are going to want as an external sub.
At that pricepoint, on a high performance/cost ratio, since you are up to building it yourself, I would recommend building an N3. You will get very good bass extension, good output since it is about 90db, great tweeter, etc. Just a great design. I have the OB7 and its great, similar in some ways to the n3, but its sealed. I have a ported ob2x, for a small room, as well (a design Danny did for another company, with similar midrange drivers) and the ported bass is quite pleasant on that one without a sub, within limitation. The transmission line, by all reports, should be better but I haven't personally heard it.
-Tony