I'm not sure how many people have experimented with heavy starbase usage, but it can get VERY powerful.  Take a look at my civilization capital:

It's getting bonuses from EIGHT starbases, giving it a 400% bonus to manufacturing, research AND wealth.  It's also in a position to get a +200% bonus to its trade-route income.  With that level of bonuses, there's little reason to build anything but farms!

Now look at my core space:

Economic starbases everywhere!  Each world is receiving bonuses from between 5 and 8 starbases, so they're all getting somewhere between +250% and +400% to all categories.  People have talked about how farms are OP, and I don't think they were even doing this strategy!

Let's look at payoff time.  Production-wise, lets say a constructor costs 40 production.  That's slightly less than the cost of a Small hull with a Constructor module and no cost-reduction techs.  Later on, you can get Tiny constructors that only cost 19 production, but for now, I'll use 40.  With 25% manufacturing, and all of that going to Military Manufacturing, you'll be generating about 5 production points per 10 billion population.  Let's than say you build a starbase assisting 3 worlds with a total of 30 billion pop.

Building an econ starbase going up to Zero-G Scaling (the best you can get in the Age of Expansion) costs 4 constructors, or 160 production.  It gives a 15% production bonus to that 30 billion pop, generating an extra 15*0.15 = 2.25 production per turn.  This will pay for itself in 71 turns.  That goes down to 35 turns if you run 50% manufacturing

Later on, things get better.  Adding in Orbital Replicators and Interstellar Collectors costs another 3 constructors, bringing the total production cost up to 280.  But the bonus goes up to 50%, so now we're adding 15 * 0.5 = 7.5 production per turn, which pays for itself in 37 turns, or 14 at 50% manufacturing.

You may need to use the two range-increasing modules (Support Field Stabilization, and Exploration Extension) to give bonuses to the maximum number of worlds.  This would bring the starbase cost up to 400, changing the payoff time to 53 turns, or 27 at 50%.

Adding the research and wealth-boosting modules costs another 10 constructors, doubling the time needed to payoff the production cost, but keeping it reasonable.  And bear in mind: once you get the +20% storage needed to fit a Construction module on a tiny hull and get some manufacturing cost-reduction techs, the cost of a basic constructor will drop below 30.

However, there is another bottleneck: starbase construction cost.  Building a new starbase costs a number of credits equal to 100 times the number of active starbases you have.  So if you have 10 starbases, it'll cost 1100 credits to build another.  At 33% Wealth, you'll get 6.7 credits per 10 billion pop, so if a starbase gives a 50% bonus to a total of 30 billion pop, that's 10.5 extra credits per turn.  That'll take 105 turns to pay for the 11th starbase, and 210 turns for the 22nd.

Trade-route boosting modules improve things.  You can get a 25% boost to the trade income of all planets with a trade route in the starbase AoE, so if you boost four trade routes worth 50 credits each, you generate 50 extra credits per turn.  That'll pay for the 11th starbase in 18 turns and the 22nd in 36 (although the later trade techs with those starbase modules are expensive to acquire).  This is limited by the number of starbases you can get providing bonuses to your trade hub (around 8)

Yet, I've found that the slower monetary payoff of starbase is not too much of a factor, because they are a great money sink.  Once you get Tourism, you'll be inundated with cash, and building economic starbases lets you spend that cash for massive production and research bonuses.  I had no problems covering my main territory with starbases, and only ran into cash-flow problems once I started throwing down a highway of military starbases with movement-cost reducers.

So, looking at the big picture: I like the challenge of positioning starbases to maximize the number of planets recieving bonuses, but I don't like how this feels like the "best" strategy.  Spamming farms on your worlds, covering your space in economic starbases to give them bonuses, and using your surplus wealth to keep building starbases feels obviously superior.  You might be able to go from +300% production to +400% on a world by building a power plant, but you're sacrificing 8 billon population that would be recieving +300% to production, research AND wealth.  As soon as you get the basic Age of War production techs, the payoff time for this strategy drops below 20 turns if you commit, making it the clear choice as soon as you've colonized all available worlds.

Changing this to be one strategy among many rather than the be-all-end-all of strategies would require either nerfing starbase modules or altering economic buildings to provide some direct increases to production, research, or wealth.  With regards to starbase module nerfs, one good idea might be to increase the construction-point cost for late-stage modules - i.e. Orbital Replicators might cost 2 points, and Interstellar Collectors might cost 4.  This wouldn't affect the impact of starbases early on, but would reduce the speed at which they get out of control, making other options viable.

So, has anyone else played around with starbases?  What are your thoughts?