The cost of any improvements are included in the Adjusted Basis of the home when it is sold.

Adjusted Basis = Purchase Price + cost of improvements made prior to the date of sale.

Sometimes, Turbotax asks for "purchase price" when it really should ask for "adjusted cost basis."  You can enter the adjusted cost basis in the spot for the purchase price. 

Exactly "how" you do it, depends on where you are reporting the sale in the TurboTax program. There's four possibilities. What section you're reporting your sale in, depends on a number of factors such as if it was a rental property in the past, or if you claimed a home office in the past, or if it was your primary residence in the past, or if it was rental property at the time you sold it.

SO where in the program are you reporting the sale? The possibilities are:

Investment Income.

Rental and Royalty Income (SCH E) (if the last tenant to move out prior to the sale was a renter in the same tax year of the sale)

Sale of Home (Gain or Loss)

Sale of Business Property

For the 2021 Tax Submittal, I am entering it as Investment Income.  So, do I enter the actual purchase price from 1998 and then have to separately add the improvements in another screen entry, or can I just add the improvements I made over the years to the original purchase price and enter that value in ONE spot?

Add the improvements to the original purchase price and report it in one spot as your cost basis.