Cable is actually a type of DSL, but here we'll talk about phone-line DSL. There are many, many versions of phone-line DSL. IDSL, CDSL, and DSL Lite can provide about *up to* 1 Mbps for absolutely no more than 18,000 feet (2.5 miles). VDSL has an "up to" bandwidth of 51.84 Mbps, with a maximum distance of 1,000 feet.
So basically you have a DSL connection between your house and the end of the block. At the end of the street, your DSL connection ends at a DSLAM, where it's connected to a segment shared by you and your neighbors.
The SLAs have three costs associated with them. As you suggested, they may have higher repair and maintenance costs, because problems get fixed faster. Rarely, the ISP may have to pay out on the SLA. More importantly, bandwidth usage is very peaky - demand is much higher than average for short periods. If I have an X Mbps line, I can use that to provide X Mbps to 1 customer 100% of the time, or X Mbps to 10 customers 99% of the time, or X Mbps to 40 customers 90% of the time. In other words, the ISP can serve many more customers per fiber if congestion is allowed occasionally, during peak periods. There's a big cost difference between "you'll have 10 Mbps dedicated to you all the time" and "you'll have 10 Mbps most of the time, but it may slow a bit during peak hours".