Allowable adjustments to cost basis are listed in publication 523.
https://www.irs.gov/pub/irs-pdf/p523.pdf
The most important one for you is improvements; improvements to real property include anything that is permanently attached to the land (or in your case, under the land) and which either adds to the value of the property or extends its useful life. A septic system would certainly be an improvement that raises your basis.
Certain closing costs may also raise your basis, but many don't. Inspections and permit fees associated with installing the septic system are included in the cost of the system.
The appraisal has nothing to do with your basis or gain.
With that being said, you will likely show a small loss rather than a gain. Losses on personal property aren't deductible, and you will have to report the sale on your tax return so the IRS has the figures, but it appears you won't have a taxable gain.
