What you charge is your business. But I wouldn't touch a 3500 word article for less than $350 unless I know the topic so well I could write it in my sleep. In that case I might charge $300.
Figure how many hours it will take you to do the job and multiply it by your hourly rate and that's the price you charge.
You mentioned training videos....are you writing to some formula? Looks like that would be harder than the inverted pyramid or pyramid style of writing.