Re: island with sink
You place the sink in the manner that is most efficient to the way you use the kitchen or island space. If it's centered, it gives you space on either side to place items as they are to be washed/washed. If you place it on the end, it will give you more open counter space, but limit your side to side area around the sink.
As to size, I would shy away from a small sink, again it limits it's usefulness and if you're going to go to the expense of installing one, make it big enough to be useful for more than washing a veggie or rinsing your fingers. It doesn't have to be quite as large as the main kitchen sink, but being the same size certainly isn't going to hurt anything either.
Cost? Depends on what it's going to take to install plumbing to the island, other than that the cost between a small sink and large sink is minimal. The cost of a sink vs no sink is probably around $1000 to $1500 depending on what it takes to tap into your existing supply and waste lines. Value? Depends on how you use your kitchen. If you're an avid cook or entertainer, then the added convenience of a second sink will be a God's send. If you're more of a frozen dinner kind of guy, then it's a bit silly to spend the money on such things, go for the big screen tv instead.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!