A spaceport can provide the benefits of a :
Refuelling station (no workers)
Ordnance transfer station (no workers)
Cargo transfer station (no workers)
That is, if the chart on page 12 of the changes list is correct about those facilities having no population.
However consider than in C# its workers that generate wealth, not total population, your shipyard is generating 100 wealth per annum with that million population, making it worth 3.3 Financial centers.
A spaceport does the work of 4,000 BP of facilities but only costs 3,000 bp. Quite a bargain if you can afford the workers.
That works only if the colony cost is zero. A spaceport on a planet with a colony cost of 2.00, which is where most near-habitable planets that are good terraforming candidates sit, will have an effective cost of 3,750 BP (3,000 BP for the spaceport itself, and assuming a total colony population of ten million, giving 1.87 colonists per worker, approximately 750 BP for the infrastructure needed to support one million workers).
This is 150 BP
more than the 3,600 BP needed for the other three installations combined, with the added disadvantages that the spaceport is significantly less portable since you'll need to cart around a couple of million colonists and their infrastructure with it, and unlike the refuelling station, cargo transfer station, and ordnance station, a spaceport can only be in one place at once. Apart from the fact that you need one million workers for it that could be doing other things, of course. A spaceport will not be the most preferred option for most military uses, leaving it with only one unique function - the ability to build space stations.
But that's where the problem lies. I can see a lot of situations where I'll end up building space stations at small colonies of ten million people or less, in systems where it's not viable to transport a station built elsewhere using a tug either because the intervening jump links haven't been stabilised or because it would take longer than building one on-site. Forward military bases in recently conquered enemy systems, orbital mining stations or harvesters in resource-rich locations, terraforming platforms - all of these might sometimes need to built on-site, and it really hurts when 20% of your workforce of five million is engaged in crewing a spaceport.
Then again, though, this is probably not going to be a very common opinion, since I suppose most everyone else will just prefer to use ships with the needed modules instead. The only reason I'm asking is that I prefer to RP needing to build large space stations on-site, so I'll concede on this request.