Not the cheapest but certainly the most effective.
Recently bought this after trying various glops & goos to seal threads on pressure gauges, cannot fault it, it has done exactly that first go, whereas with PTFE tape & some of the other stuff I have tried it has taken 5,6,7 attempts to get things airtight. Price, well I think it is a bit expensive but you don't need much at a time so it will last & for the hassle it has saved it is worth it overall.