Depending somewhat on your current strategies, there are many options at your disposal.
- You can perform A/B testing with some different designs and features across your network for some big wins. As your user-base grows in niches you obtain progressively more reputation and credence to your network.
For instance, try out some new features in a sub-section of one site with the aim of maintaining value-per-view, decreasing ad visibility (if your pages are saturating the content by much) and increasing number of views by creating deeper user-interactions through web services. Put a budget into trial-and-error scenarios, however small, and you'll have great news a year from now. It is crucial to be excited when you find the failures, so budget accordingly.
- If you haven't, create responsive mobile apps for your niche content. If you're not familiar with the process, add a comment and we'll go over how not to waste money during the process. Key concept: HTML5 apps are dead simple to make and run native on most smartphones. Drive these with RSS feeds from your site for easy multi-deployment.
- Distribute a micro-budget across new marketing and social media partners. For those who drive enough traffic to pay for the cost to your network, proceed further with some healthy cautions. If you grow large enough, this can be someone's job in a scalable business.
- Perhaps one of the best, and one of the oldest: Since you're in niche content markets, you have highly targeted traffic. Seek out advertisers / agencies to negotiate more directly with and fill your AdSense stock with higher-value campaigns. Keep pushing the bar on this. Relentlessly. Eventually you'll have much, much higher per-view and per-click payouts.
- As a network, create more cross-talk and smart "related to" feeds. The smarter the relations, the more profitable. Consider creating user accounts which span the entire network instead of one niche. StackExchange is a great example of this but you probably want something simpler with OpenID and the big social logins. Are you using Disqus or another commenting system? Bring this in house if you can afford it or pay them to deliver that content server-side.
- Combine some vacation travel with niche-events. Direct coverage of events held in specific localities is great if you can keep it up.
Make sure you're in touch with some great UX designer, and don't be afraid to try out new ones on smaller projects. Potentially the same for developers (like myself) if you get heavily into interactivity.
These are ways to grow niche-content businesses, however there are some other ways to spend your money which can still support your business.
- Experiment with near-your-niche product and service development. Can you create a game or an app cost-effectively which would be just perfect for your sites? (and/or just put Amazon products in a sidebar)
- As your total network grows into more niches, afford some budget for market research on the next best niches based on your momentum.
- Spend heavily for a few quarters on increasing the brand recognition and reputable image of your total network, as opposed to individual content sites.
- Invest directly in other businesses you may wish to partner with or eventually acquire.