Lett said donations increased by 15% for the months of June and July 2015 before returning to normal. This leak says there was an $30 million received. That would indicate that JWs collect something in the region of $100 million in donations per month, or $1.2 billion per year. That would mean each JW contributes around $150 per year on average. That may seem very low, but many JWs live in poor countries. So it might be about right.
For comparison the Seventh Day Adventist Church received $3.2 billion in donations in 2016. That works out at $160 per member - slightly more than JWs. Which is impressive when you consider that many Adventists live in Africa and other poor regions, while they are virtually absent from most of Europe. No doubt their belief in tithing and pro-education stance explains higher donations.
Edit: I didn’t think this message made it, hence the duplication. Or repetition for emphasis, TMS terminology.