About how much would replacing a septic tank cost me

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.

You can pretty much do anything above a septic tank, as long as you aren't going to be doing major landscaping that requires tractors, heavy equipment, etc..

The one thing you do want to be aware of is the location of any inspection pipes, or manholes that might have a negative effect on mower blades. Trying to mow around these nice yard "features" can be quite frustrating. For our old system, we mulched around the area containing the manholes and inspection pipes (~ 4" white pvc pipe with a cap), and planted a nice flower garden around it. We had our 3-year pumpings scheduled in the early spring before we plant, and if we needed work done on the system, flowers are easy enough to re-plant. There are also some realistic looking fake hollow rocks you can buy- we'll probably do this at our new place, as the covers are in an area not well suited to a flower garden.