Lett stasted that contributions increased by 15% during June and July 2015 following his appeal in May 2015. This internal video says that an extra $30 million was donated as a result of the appeal. This would suggest that WT usually collects something in the region of $100 million per month in donations, or $1.2 billion per year. That would mean JWs only contribute $150 each on average per year. Which seems very little, but plausible considering many JWs live in poor countries.
For comparison Seventh Day Adventist collect $3.2 billion per year in donations, which works out at $160 per member. However it’s worth noting that Seventh Day Adeventist membership is much more skewed toward poor African countries than JWs. Adventists are largely absent from most of Europe, for example. So for them to contribute more per member than JWs is actually quite telling, and no doubt a result of their belief in tithing.