The legendary (late 2014) K80 is dropping rapidly in price. Often at around UK 300 pounds and not much more in the US in dollars amazing to see a 24 GB card at the price of a GTX 1080, more so considering the street price back then was over $4,500.
I have noticed that although I have two free credit vouchers added to my account, Google is debiting the credit that expires the latest. Plainly this is extremely unfortunate and not good practice. Therefore you may well have credit expiring in a few days (or weeks) but unable to use
I am going to be controversial here, because my advice runs contrary to most peoples, but it may be useful to some. The usual advice is to code up an MNIST CNN and see it work its magic, well it may well be magic to you, hence a step back