What are the factors in determining the cost of a septic system and what can I do to decrease it

To some extent, this is going to depend on the lids required by the manhole material, and local codes. In our area, for example, both concrete manhole covers, as well as bolted down plastic covers are acceptable. That being said, either way it shouldn't be terribly expensive, if it is only the lid that needs to be replaced. For example- I just had a pump tank and elevated drain field added to my system. The tanks that were put in had plastic corrugated manholes, and I opted for plastic lids (the manhole can then be brought down to ground level, and you can mow right over the lid). IIRC the lids themselves were $75/each or so. They just use a 6" lag screw that goes into the plastic manhole (if it is a matter of simply replacing existing plastic lids, you can probably do it yourself).

From my conversation with the installer, he said that using the concrete lids (same type that we have for the primary tank) would be around the same price. I have no idea what the old steel lids cost, but I would guess they are a bit more pricey.

In terms of the labor- if it is just replacing the lids, and no work needs to be done to the manhole, I would guess an hour job at most. If the existing lids are plastic- just unscrew the lag screw, pull the old lid off, put the new one on, and screw down the lag screw. If it is a concrete lid, just dig up the lid (ours were a couple inches under ground, except for the PVC pump tube), pull up the handles, pull off the old lid, place in the new lid. Throw the dirt/sod back on top.

Tooth paste and soap (even a little anti-bacterial) are certainly fine. Basically, you just don't want to put any harsh chemicals in there that will kill the bacteria eating away at the waste.

I would avoid food and thicker items as they will not break down. I try not to allow hair, coffee grounds as they might not break down. I would definitely avoid paint.

Also avoid paper products that wont break down. Feminine products can cause havoc. Almost all toilet paper is septic safe these days, but read the packaging to be sure.

If you just bough a new house, or built one, I would recommend you have someone come out and pump the basin sooner that later. That way you know for sure that the tank is clean and in good working order and you can have a fresh start. I have heard way to many horror stories for new builds where the contractors used the tanks as a trash can and that later caused problems. Then remember to have the system flushed out every 3-5 years based on usage and local laws.