I don't think there is such a thing as an ideal gap - however, you shouldn't be basing your decision on whether you have a second child or not on the fact that your first may be jealous. You have to think about all the factors involved - do you as parents want more than one child? the counteract to them being jealous is that they have someone to play with and share with.
My two boys are 13 months apart - sort of planned that way - whilst it was very difficult juggling everything when they were babies - and yes expensive with 2 in nappies at the same time - however, now that they are 6 and 5 we are reaping the benefit in that although they fight from time to time, they get on very well - like the same things and have now chosen to share a room - When they are grown up, they will be able to share the laughter and stories of a shared upbringing. Because they are so close, we don't have the older one being responsible for the younger which makes it more difficult for us, but fairer on them.
I do think that it made the older one missed out a bit on his babyhood though and I sometimes feel guilty about that, but he cannot remember a time when it wasn't the 2 of them.
We still do things with them seperately, but they are both of an age to go to the same clubs and things and always have someone to talk to. They also have seperate friends and shared friends.
I have also spoken to friends who have had them with a bigger gap and that's what worked for them, but I also have friends who felt when their child got to 3 that they had left it too long to have another as they had got used to the increased freedom of no nappies and buggy and didn't want to go back to that again.