Well for social media is thin $2-$5 per like plus share is decent, for blog post reasonable reward is from $50 to $200 depending on quality and content, for translation it varies but $200 to $400 will be good and lastly for signature campaign a reward of $200 to $800 per week is considered good depending on the member's forum rank.
This is the thing here! People will tell you what they feel is decent but the reality is that unless you want to start coming up with your own campaigns for your project only, this is not something that will work. Every campaign tend to raise certain amount for certain period of time, and that is always their target with a certain percentage of the total amount being allocated to bounty, in the case where you are now trying to introduce payment in USD instead of stakes and then calculating at the end, then you probably want to be introducing limit of the number of participants per campaign which is how I only see that plan of yours working. Staking is used to be able to curb that main issue.
