I was able to beat the game on my first try, but with one life left. I don't remember the score though, sorry!
As far as pricing goes, I usually abide by the dollar per hour rule, so a game with 1-2 hours of content would be worth around 1-2 dollars.
But, of course, you have to take into account steams refund policy, where people can refund if they've played for under 2 hours.
I would just try to have fun making the game, and decide on price (if any) at the end once you know what you've got!
Best of luck!