The minimum contribution needs to be greater than transaction fees (credit card companies need to be paid) and your own costs. A minimum transaction would need to be a little higher, perhaps $0.50 or $0.75.
That said, development costs are not in the hundreds or in the thousands. When estimating the costs for a startup, the standard formula in the US is $10,000 per developer per month. So three developers for three months would be approximately $90,000.
You're talking more about a hobby project, so it is more like a donation box.
No, your idea isn't very new. It has been used for donations and post-development payments for things like the Humble Indie Bundle where the money goes to a charity. It just is not practical to run a business that way.
Lets try to explain again. A gamer would say contribute $10 to the microfund, and gets say 100 coins ($0.10 per coin). He then goes through and sees different projects some he gives 1 coin to others, he gives 20 coins to. Some would have perks, if you contribute 50 coins, you get into beta, etc.
So, yes the transaction fee would have to be considered on $10, but not on the individual coin contributions.
In regards to amounts raising, yes, it could take a lot, but as you mentioned Humble Bundles (for finished games) has raised millions at $5 averages for 5 games. So, if scaled, this could raise significant money.