Cost effectiveness of a T1 typically relates to the services carriers put across the T1 in order to justify the expense. It typically isn't cost effective for a CLEC to have a T1 with 4-6 lines on it. They more channels in use, the more revenue per T1, the happier the bean counters are.

PRI will give you effectively 23 phone lines. Some carriers offer a fractional PRI service for customers who don't need all 23 channels. DID's are nice, especially if 70% of your traffic is inbound.