Those seem to be the differences between their promotional and regular prices.. Not a difference between the Internet tiers only. They do run bundle promotions all the time, however, many are just for new customers, or when upgrading. Some are time limited, like your first example is "12 Month Promotion" price.
HSI and Blast speeds vary depending on markets.. I believe in my area they are:
HSI/Internet Plus is 25-30 Mbps ($69.95 default price, discounted by $15 when bundled with other Comcast services)
"Blast" starts at 50-60-75 Mbps, many times offered with bundles for additional $10..
Just call and ask for current promotions, tell them you want to downgrade to about half of what you're paying and see what offers you can get. If you can't get anywhere that way, just downgrade to the lowest tier for a month or two and call again. Keep in mind they also have $39 charge for "change of service" I believe.
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