You are a:
Some of them would say I'm half a duck
A synonym for duck is bob. I would consider ducking something thrown at you to take more time than a bob. So, I guess a bob might take half the time as a duck...? ;-)
I may be used if you want to try your luck
Bobbing for apples or cherries.
I inspired your sweetest dreams, will say the most accurate
The Eurythmics wrote and sang the song "Sweet Dreams" shortly after the break up of their prior band called "Tourists", who had a member named Robert Görl. Robert is often shortened to Bob.
But for only 15 of them, it was worth a rate
In the classic Charles Dickens’s “A Christmas Carol,” Bob Cratchit makes 15 shillings each week.
Some of them tend to collect me
The UK changed their currency in 1971, so the shilling (aka bob) is no longer in circulation. As such, they are probably only kept by collectors.
A bobbin might be collected by tailors/seamstresses or DIYers/crafters for various reasons. I know some people use empty bobbins as flatcar cargo in model railroading.
Even if I'm usually just worth a few penny
The shilling (bob) was one of the smallest denominations of currency the UK had, so were only worth the equivalent of a few American pennies. The current pence is also referred to as a penny and is also the smallest denomination for the UK.
Even if this isn't correct, it sound good.... right?