Ask them to do it themselves in a spreadsheet?

Might depend on how old they are and if you can assume they have spreadsheet access, but something they do themselves will likely have more impact....

ETA: If a graph of balance vs. time isn't needed, they could do the P * (1 - load) * (1 + (i - fee))^n calculation on a calculator for a one-time contribution. Need to use the Future Value function (or get into its derivation) for annual contributions.