In general, it depends. There are countless ways to get leads and it is necessary to understand each particular tool’s benefits in order to compare them and prioritize.
Some instruments have low launch cost. Others are more difficult to set up. Some instruments give good results from the start. Others need time to heat up. Some instruments will give sustainable flow. Others work in a burstable shorterm manner.
Moreover, even within instruments, cost per lead will vary depending on the offer and the audience.
Bottom line is, there’s no general rule for how much leads cost. What you can really do is to research your industry best practices, get baseline results with the classics and then carry out tools, channels and strategies your competitors are not so strong yet.