Is the one who says, "Astaghfirullah (I ask Allah for forgiveness)," rewarded with a good deed, or is a sin erased from him, and is the reward dependent on ritual purity?

Seeking forgiveness (Istighfar) is one of the best forms of remembering Allah Almighty. It is permissible to do so without being in a state of ritual purity (tahara), but remembering Allah while having ablution (wudu') is better and carries a greater reward.