When you think that the New York Metropolitan Area has a bigger GDP than Russia, what you are suggesting (real city states) starts to actually make sense.
Depending on how you define it, the NY metro area extends into NJ, CT, and PA. I still don't think it surpasses Russia, but it does surpass the vast majority of countries, including Spain: http://www.usmayors.org/metroeconomies/2013/201311-report.pd...
I'm not so sure these are very good examples. Monaco is effectively a tiny gated community of France for the super rich that produces little-to-no goods or services. Luxembourg is heavily dependent on support and the existance of the EU (receiving approximately €3,000 per person, the highest in the entire EU). Singapore is probably the best example of a city-state succeeding outside the framework of a larger state, but the size of their public debt is a little frightening for such a small country - and would be slightly less so if it was within a larger body.
Monaco and Luxembourg both are inside the EU and exist very much as a result of cooperation with France (and the Treaty of Utrecht). They both have ceremonial militaries only, and subsist by facilitating tax avoidance for the nearby elites.
