You specialise in (and have to pay for) just the one specialisation in ectomancy, not every sort of ritual that could conceivably be performed by an ectomancer. They key is that you can specialise in a theme or a type of thaumaturgy – ectomancy is a theme and specialising in it will give you the bonus regardless of the type of ectomancy you are doing. Otherwise, you can specialise in a type of thaumaturgy such as summoning or wards, which will give you the bonus regardless of whether it's a demon, ghost, or something else. You don't have to specialise in ectomancy and then also specialise in each type of thaumaturgy that ectomancy could conceivably apply to.
You get to choose one specialization bonus for free when you take Thaumaturgy (the Stunt; YS p. 181). If you want to specialise in ectomancy, take the bonus as a complexity bonus or a control bonus when performing ectomantic thaumaturgy. This gives you that bonus whenever you are performing ectomancy. It's that simple.
If you want more specialization bonuses, you need to take Refinement (YS p. 182) for each bonus you want beyond the free one you get with Thaumaturgy. Each time you take Refinement you can choose another bonus (control or complexity) in whatever type of Thaumaturgy you like. You can improve your bonuses when performing ectomancy, or you can branch out and improve your other types of thaumaturgy. (Just keep in mind that your bonuses have follow a column structure, as described under Refinement.)