Costs can vary per neighborhood(unless its a special deal/etc) and available speeds can vary house to house in a single block.
The reasons speeds vary is both SNR and capacity. If they are over capacity for the node serving your area, they are supposed to only offer the lowest speeds where they can maintain roughly the lowest guaranteed speed for the plan. a 10mb/1mb plan MIGHT only guarantee 6mb/256kb speeds, but that MIGHT not count peak hours where they likely don't guarantee anything at all.
SNR is affected by how far you are from their equipment. Signal degrades over distance over a copper line, which is the majority of cable internet connections. even if it is fiber to the neighborhood, its typically copper from there to your house. So if you are the farthest house down, you may only be capable of the lowest speed plan because SNR is so low.
Typically they will offer the highest speed they are told they can, and work from there. The technician will come out, test the line, and you may end up working from there down to a lower speed. Some of them might have tools now where they know roughly what SNR should be to your house, but you don't know for certain until you have a modem to test it.
You never know for certain unless they hook it up. Most of the time it will be fine, but I was on a DSL support line and sometimes it doesn't go well....lol