iPhone 3G not turning on

I really need help. I use my iPhone everyday so I really want to fix my iPhone asap...
Basically, my iPhone right now doesnt turn on properly. Every time I press power button, the apple logo shows up then 30 seconds after, it shuts off.
All I did before having this problem was power off my iPhone but it froze on the spinning wheel screen so I held home and power....
I tried everything to try to fix it... I held Home+Power numerous times. I tried putting it into recovery mode, and that doesnt work. I tried DFU mode - nope. I tried Home+power then release power, holding onto home - that didnt work either.
I have no warranty on this iPhone but Im going to visit the Genius bar in 3 days if I cant revive this phone. Im afraid repair costs would be really high and I end up not fixing it - thus eliminating a significant belonging of mine.
Please provide some advice on how to fix my iPhone. And if you know the repair costs, Id like that information too. I just need some sort of idea what the cost range is going to be.
Thanks
iPhone 3G iPhone OS 3.1.2