How much does it cost to generate leads?

Costs vary depending on who your clients are and what form of lead generation you use. If you choose PPC ads, they can be very expensive even if done properly. If you choose organic SEO, it's free! But the results may take years to show up in your analytics or actually, can never hit through competition. It all depends on what your goals are for the business and how much time/money/effort you're willing to invest in each channel.

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.