Dapps will pay a nominal fee to register a side chain on the main Lisk blockchain.  After that small registration fee, there are no continuing sidechain payements to the Lisk Foundation.  As usual, correct me if I'm wrong, Max.

As sidechain apps charge for the services they are providing, they will probably ask for Lisk as their payment, since that is easiest to do (although writing for example Bitcoin processing directly into a Lisk app is theoretically possible).   Sidechain users will have to purchase Lisk at an exchange to use those sidechain services.   This will result in upward buying pressure for Lisk as a coin.  

Take for example any future Lisk sports betting dapps.  Lisk would effectively be the casino sports room tokens used among gamers to settle their wagers.  Those tokens would have to be purchased on an exchange, and could be later "cashed out" at an exchange for other currencies such as BTC and eventually USD.

Vegas, baby.