$20 · Texas Instruments BA-35 financial calculator

Used excellent condition It features cash flow analysis and time-value-of-money and compound-interest calculations. It can also recall time-value-of-money values as well as annuity operations. It converts annual percentage rate (APR) to effective rate (EFF) interest and does amortization balance payment-to-interest cost-sell-margin and markup calculations. The the BA35 runs on solar power you'll never have to worry about replacing the batteries.