Is it safe to plant shrubs or flowers in the soil above the septic tank

I found some rough estimates at costhelper.com

Installing/replacing a conventional septic system (including the tank) averages $2,000 -$5,000 in the Midwest, but can be $4,000 -$12,000 or more in areas where materials and labor rates are higher.

Enhanced, engineered or alternative septic systems that use mounds, sand/peat filters, aerobic systems and/or constructed wetlands can cost $10,000 -$20,000 or more, according to the Rhode Island Regional Water Quality Program. These alternative septic systems work better than the conventional approach for sites with high groundwater or slowly/rapidly percolating soil, or near drinking water supplies, wetlands, coastal ponds or other water resources.

The cost of a septic tank alone runs about $500 -$1,800 depending on size (ranging from 300-1,000 gallons) and type. Piping and other needed items adds another $100 -$200 to the total cost of materials.

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.