A tripwire is a low cost product or service that links the email bait to the core offer.
The price is relative to the core offer and is typically 1-10% of the main offer. If the core offer is $497 the tripwire would typically be between $5-$50.
The goal is not to profit from just the tripwire but to lead into upselling higher priced products or your core offer and to change the relationship from a subscriber to a customer. Someone who has already bought is far more likely to buy again and even straight away. Indeed typically 10-25% of people who buy an introductory offer purchase the next (core) offer in a funnel.
By showing a low priced "tripwire" offer straight away after the optin you increases the number of customers buying your core offer you sell directly afterwards (on the very next page).
The tripwire is designed to solve one specific problem really well and like every great offer you need to get across what the before and after state will look and feel like for your prospect.
Tripwires that do well are online courses (related to the core issue in the lead magnet), one off services such as audits or specific coaching calls, full length books, standalone software (i.e. wordpress plugin) or physical products (such as ski wax…).
The price of the tripwire needs to be so low that there cannot be any rational objection and that the value to price ratio is off the charts. This means that you’re not going to make money with the tripwire itself but it can cover a chunk of your ad spend to get someone into your funnel who is way more likely to buy your core offering.