Code p0455 diagnostic code & check engine lightbulb replacement cost?

-my car (2004 cavalier if that matters..) failed inspection because i got a diagnostic code "p0455 evaporative emission system leak detected (large leak)"

what could have caused this?

my mechanic (who i fully trust) said it could be

1. filling up on gas with the car on (which i did once accidentally)

2. not putting the gas cap on correctly (which i also did once, i thought i closed it tight but the next time i got gas, it just fell out when i opened the little door thing)

but my question is, is there anything else that could have caused the code besides the two things mentioned?

also, either the fuse for my check engine light is blown or the blb went out because that isnt working either and i have to get that fixed too. how much would that cost and about how many hours labor? thanks!