You can probably do it, but your competitor who do target users will end up with $30 CPA and you will get $300 (or worse).
So, unless you are ok with x10 user aquisition cost, you will quickly stop doing it.
what are you talking about?
Lets say you are running SaaS for Azure admins. Do you want to buy the top banner on slashdot.com for the whole month, no targetings at all, for 50K/month, or you only want to show yor ad if user was interested in at least 2 stories with azure tag, for 5K/month?
Automatic (aka highly targeted) ads are more effective, not x1000 less.
And your competitor can always claim that 10% of client aqusition cost is donated to fight climate change... and then you stand no chance ;)