Just come back to this and interested to see the turn the discussion took. I do think that age gaps and natures of the individual children will affect how difficult things are. The age gap thing isn't straight forward though. In a lot of ways I'm very happy with the 4ish yr gaps we ended up having both times. Going from 1 - 2 was a breeze for me because 2 was a very easy baby who slept through the night 12 hrs from just a few weeks old and was just very easy going and not demanding. My first had reflux and so had been much harder work so the combination of an easier baby, me knowing what I was doing, and my older one being old enough to be quite independent made it all very easy. In fact easier than having just 1 in many ways because my eldest would entertain the baby for me so I could get on with stuff.
Perhaps that is why going from 2 - 3 was more of a big deal to me? On some level I'd obviously expected a repeat of the experience of going from 1 - 2. A bit more washing but nothing major. The truth was that I got a baby that had worse reflux than the first one. I put myself under huge pressure to BF because #2 has a nut allergy. The baby didn't sleep through until he was 9 months old and is still pretty hit and miss now at 18 months (I've been up a couple of hours in the night over the last 2 nights). And fitting a baby in with an 8 yr old and a 4 yr old is a whole different ball game to fitting in with one 4 yr old. Immediately the baby was born I had to miss out on a production my eldest was in and a big event at pre-school that the middle one was involved with. Then I have to take the baby (now toddler) with me to swimming lessons and hang around for ages as the big two have lessons at different times. My eldest has a hobby that involves hanging around for hours every week in a toddler/small child unfriendly environment. Initially that meant looking after a baby and keeping a small child entertained. Now middle is more easily kept busy with a DS while waiting around which is fortunate because I'm very busy trying to stop the toddler tornado from doing himself serious harm.
I wouldn't change it for anything. I had a very strong feeling that someone was still missing from my life until DC3 arrived that he has definitely filled that gap. He is amazing as all our children are and I couldn't imagine life without him. That doesn't change the fact that it has been really hard work though. I'm sure for some people that number 3 does just slot in, like number 2 did for me, but I don't think it's a safe assumption to make when deciding if going for it is the right thing to do. I just think it's a lucky bonus if that does happen for you!
Also re the expense thing. It's true that babies don't have to cost much but they have a habit of growing into children that do! Even if you don't think extra curricular activities are important (and OMG the way the costs of those stack up is horrifying) they are still going to need to be fed, shod, clothed, schooled and entertained. School dinners for 2 costs me £21 a week. All three needing new shoes (and trainers for the big ones, necessary for school) nearly makes me faint at the cost (even though we can afford it). None of mine seem to be able to wear cheap off the shelf shoes. I can never get them to fit. Then school seems to demand money left right and centre. There are school trips, charity dress down (or up) days, costumes for theme days and plays, Christmas parties and discos and dinners, tickets to performances and so on. It also never ceases to amaze me the amount of food a small skinny boy can put away. BF is free but you can't expect them to live off that until they are 18...