Membership fees in the graph above were drawn from the QST covers available at arrl.org's archive search.
Based on the fees in the first graph, the second graph shows how many years it took to reach the break even point. Complete data ends at 1990. The next few years assume a continued $39/year, and there isn't enough info to hazard a guess beyond that.
Now you can decide if Life Membership is worth it for you. Keep in mind
money isn't the only reason, but it's the easiest one to graph!