Hey that's a great question.
Here is what I would do with $300.
1. I would be certain I have an offer that is already converting. The only way to know is if your Affiliate manager is being honest, or if the network provides EPC numbers, or if you use spy tools and see this product being promoted for a long time.
2. I would test 2 - 4 landing pages tops...Again to do well here I would not be creating a lander from scratch. I would already be seeing what others are doing.
3. I would hit up the ad manager at the native ad platform and ask them best placements fr the offer I am running. They should be able to help. Tell them your initial budget and that if you cant make it work then you will go somewhere else but if they can help you - you have unlimited spend potential. Maybe that will be enough for them to give you some time.
4. Test about 5 - 10 ads all at once on about 10 of the top placements... Even at that rate you are only allowing $30 per placement which is pretty slim...