You got it wrong. Data roam is feature of prepaid sim card regardless how much did you pay for it. Read here
http://www.starhub.com/personal/mobi...aid-cards.html
http://www.starhub.com/personal/mobi...appy-roam.html

So SIM card comes with with variety of prices, starting from $8 and above - $8 SIM card comes with $10 credit, $15 card comes with $18 credit, $32 card comes with $32 credit.

Now data balance is separate matter. Some SIM card purchase options come with data balances, some - cheaper options do not.

For example if you buy $8 card, you can use whole $10 credit to buy 1.2GB data, in this case you won't have any credit left for local/international calls or SMS, but if you are not interested in these - you are fine, you will use data only. Another example - if you purchase $15 SIM with comes with $18 credit then you can use $15 credit to buy 2.4GB of data and you will have $3 left for calls/SMS/etc. Data purchases can be done using codes or via happy prepaid app.

That purchased data is valid not only for Singapore but for other countries as well. You do not need to purchase a special SIM to use in these countries or purchase some other option for it - you just need to set correct APN settings - it can be done by downloading starhub happy app and inside it it has an option to configure phone.

Hope this clarifies the situation.