A taxable capital gain is the difference between your selling price for shares and the price you paid for the shares. Your purchase price for shares is called your cost basis. Reinvested dividends are taxed when earned and then add to your cost basis in the investment for capital gains purposes.For example, you invested $1,000 in a mutual fund and have earned and reinvested $50 in dividends. As a result your cost basis is $1,050. taxable capital gain is the difference between your selling price for shares and the price you paid for the shares. Your purchase price for shares is called your cost basis. Reinvested dividends are taxed when earned and then add to your cost basis in the investment for capital gains purposes.