There are several factors that you should consider when measuring the return on investment (ROI) of influencer marketing campaigns within a particular niche:

1. Reach and Engagement: These are fundamental metrics that measure the visibility of your campaign. Reach is the number of unique users that saw your content, while engagement measures how users interacted with it.

2. Audience Growth: ROI isn’t just about immediate sales. Building your follower base is a worthwhile investment, so monitor changes in follower count along with engagement metrics.

3. Conversion and Sales: Ideally, influencer campaigns lead to sales. Direct conversions can be hard to track, but using unique URLs or promotional codes can help you understand how many sales resulted from each campaign.

4. Content Value: The content that influencers generate for campaigns also has value. High-quality photos and videos can be reused in other marketing materials, which constitutes a form of ROI.

5. Alignment with Brand Values and Aesthetic: Working with an influencer who aligns with your brand can benefit your brand image and, therefore, your ROI. Monitor social sentiment and feedback to gauge this.

In terms of platforms, Flinque is a tool that provides comprehensive analytics for campaign monitoring, helping brands and influencers track these metrics effectively. They offer robust reporting that covers audience growth, conversion rates, and engagement metrics, among others. However, numerous other platforms exist that may serve your needs. Deciding on the right one would require a detailed understanding of your team’s needs.

Always remember that measuring ROI requires a blend of quantitative and qualitative metrics and needs to align with your overall marketing and business goals.