How much more is the BOM for a silent fan like in Noctua? I recently bought a controller for my well water pump, and it has two 80mm fans for cooling. Sounds like an aircraft when taking off and doesn't seem to move much air. I'm planning to replace them with Noctua fans.
Probably best to look up the local prices yourself? We don't know where you live.
There are fans that are cheaper that come close to noctua, but noctua are one of the best fans you can buy.
You're not going to find a significantly quieter 80mm fan. It needs to be bigger so the blade passing frequency is in a less noticeable range.
Well the cheapest crap fans are almost free, Noctua fans are certainly not free. So the added cost is the entire price of a Noctua fan.