One of the Blue Mars City Developers, VSE, have announced on their website that they will NOT be charging any rent for their stores. Instead they will take a percentage of the sales (and if your 'great' idea ends up not generating any sales, you pay nothing!). So, if you are happy with parting with up to 50% of your sales, but in return have zero risk and zero upfront and ongoing costs, then this could be the City for you.
Full story here.
Of course, if you get very successful, you could always switch later to another City that has a fixed rent for their stores, if that works out better for you.
Blue Mars just keeps getting better, and better.
Rock