A T1 is about $5-600 around here... to have it as a PRI is another $100. If you are considering a T1 and can support it I would recommend you go to PRI vs anything else. Always have better control and options. Not sure i'd recommend a PRI for someone with 6-lines tho. Kind of a waste.

Be better to get a T1 with 6-lines for voice and the other 18 channels as DIA. That makes a lot of sense.