On septic here-
-life span of the drain field is appx 30 years if it has been taken care of (replacement cost of field is appx $7500, at least that is what it cost us). We got 37 years out of the field.
-clean out runs about $550 includes pumping, pressure washing, jetting lines both ways, camera inspection and dig charge (the tank is about 6 ft down), clean out should be done every 3-5 years.
-never have used the additives
Be wary of the low price specials you may see. The end cost will probably be significantly higher with all the add ons.
For our area, septic is much lower cost overall. By the time sewer is added to water bills, it is usually 4 times higher then what our water bill alone is.