Say you have 10 machines at SoftLayer and use 30TB a month. Each machine comes with 3TB and you pool your bandwidth for $25 per server so you can allocate the whole 30TB to your proxies. It's unknown what fraction of your server cost is applied to bandwidth, but we know the point where you start saving.
At Amazon 30TB of US-East EC2 bandwidth costs 10000 x .12 + 20000 x .09 = $3000.
If you estimate the bandwidth portion of your SoftLayer server cost at less than $275 you're saving money when using your full bandwidth allocation. With servers starting at $159, sub-$100 seems realistic.
In our case with dozens of servers and ~60 TB of bandwidth, we're saving thousands a month compared to EC2.