The amount you can purchase for a recharge in Cuba is limited to a certain rate in CUC, set by ETECSA for those recharges sent from outside the country.
The minimum for a recharge is the equivalent to 10 CUC and the maximum is 50 CUC, for cell phone recharges, as well as Internet WiFi Nauta recharges.
During Cubacel promotion period, you must recharge the equivalent to at least 20 CUC in order to benefit from the bonus.