Hello! Me and a friend will be flying into Manaus from Rio on February 7th. From there we plan on making our way to Iquitos, and taking a plane to Quito. How long will it take to get from Manaus to Iquitos? Should we wait to buy our plane tickets to Quito until we get to Iquitos?
Thank you for any help and advice!

That depends on whether you want to take the fast boat or the slow one. I took the slow one from Iquitos to Manaus and it took 2 days to get to the tripartite border, then another 2 days to get my Brazil visa from the Colombian side, then 4 more days to go the rest of the way to Manaus. So, going downstream, the minimum would be 6 days, but you should probably plan a couple more (even though you should already have you Brazil visa by that point) because there's not necessarily a boat leaving from Santa Rosa/Tabatinga every day. Going upstream will be slightly slower but probably not by a lot.

I think the trip cost me about $15 including food for the Peru portion and $75 including food for the Brazil portion (those costs are in US dollars for the whole trip, not per day).

I think the fast boat from Manaus to Tabatinga is only about one day (only a rough guess though), but the slow boat was much better for my style of travel.

I'm not sure of exact prices, but flying to Quito from Iquitos will be rather expensive because you'll have to go through Lima first. You could save a couple hundred bucks (as I did going from Quito to Iquitos) by flying to Lima, then busing to Quito. If, that is, you don't mind taking a 30-hour bus ride. You could break that bus trip up by stopping around Mancora if you want to make it more tolerable, though.