I live near IAD and just visited Tanzania, so I’ve been watching these routes for the past year. For what it is worth I’ve seen sale fares from IAD to NBO and DAR over the past two months RT in economy ranging between $600-700 on Skyteam. I also was able to purchase tickets for my family from PHL-NBO RT in Delta/KLM business class for just over $1700pp via the Expedia deal that has been discussed in the premium fare deal thread. We decided it was worth adding the drive to PHL to get that price. We just returned from that trip, but tickets at that price were still available at the beginning of the summer. So if you know what you’re willing to do in terms of airports and dates and keep an eye on the deals, you can probably find something at or near those price points.