A T1 is simply not adequate for a web company for two reasons: 1) it is a high-impact single point of failure (any issue here results in 100% outage); 2) it is limited to 1.5mpbs. I guarantee you Heroku is beyond 1.5Mbps at peak.
To make this a fair comparison, you need to look at burstable bandwidth. Unless you want to manage your own redundant BGP routers and multiple carrier lines, you'll want blended bandwidth provided by your datacenter or a specialized company (InterNAP, etc). This bandwidth will run you $50-100/Mbps/mo (let's say $75) and is billed at 95th percentile of usage. A year and a half ago when my company's EC2 network transfer costs were ~$8,000/mo (with a total EC2 bill of ~$30,000/mo), I worked out a bandwidth need of ~60Mbps (at 95th%) - this would represent a bandwidth cost of $4500/mo. Let's assume Heroku's EC2 cost breakdown is similar, and that their needs are 2/3 of what ours were then - we're left with $3,000/mo for bandwidth.
Yes, you're still paying a premium, but the cost savings of colo hosting are quickly offset by the additional manpower required to maintain physical servers. Whatever difference still remains is easily justifiable given the amount of flexibility the cloud affords you. This is exponentially more important for a startup where agility can make or break you.