I don't know where you are or who your provider is, but around here Verizon doesn't guarantee squat for business DSL, either in term of uptime or in terms of bandwidth - and I'll bet money that they're typical in that respect among other DSL/cable providers. If yours does, I would say it is by far the exception and not the rule. Mostly, if you need SLA's, you can pretty much forget business-class DSL or cable, and skip right to fractional T1...
I was on a fractional T1 a few years ago at a small office of a very large company (some contractual requirement). It cost over $700/month! Eventually the price went down to $249 or $429/mo or something. And what a pain it was when it went down - you never got the same tech twice and none of them had ever seen it before! It's replaced now with a single cable connection.