Just divide the yearly cost by the rate of interest or inflation. $10 billion divided by 3.71% yields that $270 billion figure. Use whatever mental shortcuts work for arithmetic in other contexts. There's no need for an actual infinite series.

Explaining it the other way around, if you had $270 billion now, you could fund the NSA by $10 billion per year in perpetuity based on the interest. That's a simple calculation of $270B x 3.71%.