Most of the best online singing lessons services in this guide offer a free trial (from 7 days upwards) or a small sample of free content. This gives you the opportunity to try a few before you commit.
Beyond this, once you start paying, online lessons start from around $15 per month. You can make savings if you sign up for longer periods, such as a full year. Some services even offer up lifetime subscriptions. These might seem expensive initially, but if you plan on using the content consistently and over a long period of time, could be worth considering.